At 17, you can join the army but can't buy an Instant Kiwi ticket.
At 16, you can buy a gun but not fireworks. You can also work full-time and pay taxes but don't get a say in who runs the country.
The minimum ages set out in New Zealand law are not only confusing but, as proposals to raise the driving and drinking ages show, can also be contentious.
Former Labour cabinet minister David Benson-Pope, who successfully lobbied for an increase in the minimum age for buying fireworks, says while politicians setting minimum ages do look at research, ultimately they are forced to make decisions that are politically pragmatic.
For some, like soldier Cameron Ackroyd and schoolboy racing driver Mitch Evans, age is not that important.
Cameron was 18 when he served in East Timor, while Mitch was just 14 when he competed in last year's Australian Formula Ford Championship.
The irony of being able to drive a V8 supercar at 250kmh but having to have a parent with him on the open road is not lost on Mitch (now 15).
He upgraded from a learner's to a restricted licence earlier this year but still has to be accompanied by a fully-licensed driver when carrying passengers: "It is a bit crazy," he acknowledges. "It's quite difficult to stick to the speed limit as well."
Next year, he hopes to be in Europe competing in the GP3 series and his ultimate goal is a Formula One drive by the time he is 19.
New Zealand land speed record-holder Owen Evans says his son's age is not an issue because he is mature for a 15-year-old, and his youth could in fact be a "huge advantage". Drivers need three or four years in other classes overseas before advancing to Formula One and are unlikely to be signed if already in their 20s.
Asked if he is mature enough to be racing, Mitch says he tended to take more risks when he was younger but now tries to think about the bigger picture rather than just one race or one corner. He also thinks young people can have more ability than drivers who are older.
Private Cameron Ackroyd's only concern when joining the army at 17 was that initially he could not drink and "go out on the town" with most of his friends.
His six months of training as a rifleman was thorough and, at the end of it, he felt ready for whatever the army needed him to do, says the 20-year-old Cantabrian, who is considering deploying to Afghanistan.
"I think [the minimum age for joining up] is not too early, and provides a good opportunity for many young people, especially those not doing so well or not well suited to school."
Nonetheless, some arguing for changes in minimum ages point to a growing body of research that shows the brain takes much longer to mature than previously thought.
Earlier this month, principal Youth Court judge Andrew Becroft said New Zealand was out of step with the rest of the world by treating 17-year-olds as adult offenders.
Becroft said New Zealand was breaching a United Nations convention which stated adulthood began at 18 and was flying in the face of research that showed youth offending must be seen differently from adult offending because the part of the brain that controls logic, judgement and wisdom was not fully developed until the ages of 25 to 30.
In the past, teenagers who prefer texting to talking have driven some parents to call for a minimum cellphone ownership age of 14.
Last week, the Justice Ministry suggested a voluntary accord to stop knives being sold to young people.
And, from October, 12- and 13-year-olds accused of very serious crime will be able to be referred to the Youth Court, instead of the Family Court, giving judges more options for dealing with them.
The debate about age-related policies even extends to sport, with a recent correspondent in the Otago Daily Times public opinion column asking why in some masters running events, a man had to be over 40, yet a woman only over 25.
Once, young people were given the "key of the door" at 21, but today the age when they are considered mature seems less clear cut.
School uniforms still mark the wearers for special treatment, as in many cases, such as swimming pools, movie theatres and bus fares, those still at school pay reduced prices.
But there is a considerable lack of clarity when it comes to having a punt. While even a child can play Lotto, Keno and Big Wednesday, under the Gambling Act 2003 no-one under 18 can buy an Instant Kiwi ticket.
New Zealand Lotteries spokeswoman Karen Jones says the thinking is that Instant Kiwi could be more attractive to young people, given tickets cost as little as $1, are brightly coloured and have the appeal of winning money immediately.
Under a Dunedin City Council bylaw, no-one can carry out a beautician treatment, skin piercing or tattoo on someone under 16 without the written permission of the person's parents or guardians.
Crazy Horse Tattoo owner Chris Downing helped write the bylaw but prefers to tattoo only those 18 or older.
"Some 16-year-olds are mature, know what they want, are not picking something stupid and have their parents with them," he says.
"Other 16-year-olds have no clues."
He is also concerned that children as young as 10 are getting piercings from some operators.
Social networking sites, such as Facebook and Bebo, advertise that users must be at least 13 to sign up but many simply lie about their date of birth and Netsafe executive director Martin Cocker says plenty of children aged 9 or 10 have accounts.
Young people of any age have the right to access contraceptives and abortions without parental consent.
And anyone, regardless of age, can legally smoke cigarettes and drink alcohol. This is because New Zealand does not have a minimum legal age for these activities, only a minimum purchase age. (According to Ash, most New Zealand smokers start before they are 14.6 years old).
There is also no minimum age for employment in New Zealand. Paul Roth, professor of law at the University of Otago, says the main rules on child labour are that children under 16 cannot be employed during school hours and those under 15 cannot do hazardous work such as logging, manufacturing or driving forklifts.
Even so, there is an average of one fatal work-related accident affecting people under 16 each year, mainly through driving ATVs on farms, and about 200 children a year are hurt badly enough to require hospital treatment.
Lifting the minimum age for buying fireworks from 14 to 18 in 2007 was one of the few times a legal age has been raised in recent years.
Former environment minister David Benson-Pope says 18 seemed the threshhold for many adult activities and proved "about right" because there were now fewer fires and less vandalism.
"Setting ages is really difficult," he adds. "Take the alcohol one. I've always been of the view that if you can get killed in the Solomons or Afghanistan as a soldier at 18, then it's pretty hard to argue that you shouldn't be able to go to a bar and buy a beer."
However, he does think the driving age is too low and should be raised to 18.
"Overseas, people can't believe the age limit we have for driving. In Europe, it's generally much higher."
Those setting minimum ages need to consider whether they are widely accepted by the community and whether they are policed, he says.
In many supermarkets supervisors now authorise alcohol sales, perhaps because they have been "stung" by monitoring operations, but young drivers on restricted licences routinely break the rules about carrying passengers and driving outside limited hours.
"If you've got an age limit on something, whether the decision has been made because of logic, political pressure or scientific advice, if you don't reinforce it, it's going to be ignored," Mr Benson-Pope says.
In its issues paper on the reform of New Zealand's liquor laws, the Law Commission favours leaving the minimum age for buying alcohol at on-licences at 18 and increasing the minimum purchase age at off-licences to 20.
This should help reduce the supply of alcohol to people under 18 by older friends, while still allowing 18- and 19-year-olds the freedom to drink in the supervised environments of on-licence premises, it said.
It also favours making it an offence for an adult to supply liquor to a young person unless it is a private social gathering and that adult has the consent of the young person's parent or guardian. A spokeswoman for Justice Minister Simon Power says the Government will not comment until it receives the commission's final report in mid-to-late April.
Meanwhile, Transport Minister Steven Joyce says improving the safety of young drivers is the Government's top road safety priority.
Young drivers make up 14.5% of New Zealand's population and 16% of all licensed drivers, but in 2008 they were involved in about 38% of all serious injury crashes.
To tackle the problem, Mr Joyce will take a package of measures to Cabinet this month. These include raising the driving age from 15 to 16 and tightening the restricted licence test to encourage novice drivers to do about 120 hours of supervised practice before driving solo. A proposal to reduce the youth blood alcohol limit to zero will be discussed by Cabinet next month.
All of those measures have been successful in other countries but the Government has been particularly keen to learn from Australia, where young people have a road fatality rate of 13 per 100,000 of population compared to New Zealand's rate of 21, Joyce says.
For a long time, scientists believed the human brain was fully mature by about 18, says the developmental psychologist.
But, while the size and the structure of the brain do reach adult levels about then, a more recent brain-scanning technique, functional magnetic resonance imaging (fMRI), shows functional maturity comes as late as 25.
This new scientific data has only helped explain what we have known about teenage behaviour for a long time, she adds. Adolescents are impulsive, tend to make bad decisions in the heat of the moment, overindulge in alcohol and engage in other risk-taking behaviours that increase their morbidity and mortality.
"The key is to help young people establish boundaries around their behaviour and, given the nature of their developmental stage, they're less likely to do that for themselves ... It's about adults accepting we need to nurture young people for longer than we did in the past and for longer than we're legally required to."
The Western idea that if we treat a child like an adult, they will behave like an adult is an experiment that has failed, she says, adding that lowering the age for buying alcohol was a "huge mistake".
"In any country, the functional drinking age is two to three years below the legal age so when we lowered the purchase age to 18 in New Zealand, it was really giving the blessing to much younger children to consume alcohol."
"I'm concerned about the consumption of alcohol at 18. I'm dead set against consumption at 15, 16 and 17, particularly given the knowledge we have about the nature of the changes going on in brains at that time."
On the subject of the driving age, Prof Hayne says we might want to think about whether it is okay to have a "highly risky brain behind the wheel of a very large weapon". While some argue that raising the age would make life difficult for young rural people, what needs to "lead the charge" in such decisions is the science of development.
But Prof Hayne says we shouldn't aspire to "squeeze" all risk-taking out of adolescents because a small to moderate amount of it is a good thing.
Young people who take a few risks ultimately have better developmental outcomes than those at either extreme. Instead, they need boundaries, consequences and places they can engage in positive kinds of risk-taking behaviour that don't involve drugs, alcohol and unsafe sex.
Adolescent brain development research is already shaping debates about when individuals should be considered mature for policy purposes, according to recent research published in the Journal of Adolescent Health.
For example, adolescent brain immaturity has been used to make the case that teens should be considered less culpable for crimes they commit, but parallel logic has been used to argue that teens are insufficiently mature to make autonomous choices about their reproductive health.
Authors Sara Johnson, Robert Blum and Jay Giedd noted that throughout history there have been biological benchmarks of maturity. For example, in 13th-century England, when feudal concerns were paramount, the age of majority was raised from 15 to 21, citing the strength needed to bear the weight of protective armour and the greater skills required for fighting on horseback.
More recently, in the United States the legal drinking age was raised to 21, whereas the voting age was reduced to 18 so as to create parity with conscription.
"The variation evident in age-based definitions of maturity illustrates that most are developmentally arbitrary. Nonetheless, having achieved the legal age to participate in a given activity (e.g. driving, voting, marrying) often comes to be taken as synonymous with the developmental maturity required for it."
A separate but related issue is that of "delayed adulthood", a relatively recent phenomenon in which many 20-somethings put off the responsibilities generally associated with adulthood, such as marriage, children and mortgage payments, to travel and pursue their careers.
The trend has been linked to everything from economics to an unwillingness by today's youth to grow up. But Prof Hayne sees it as a natural consequence of the increasingly complex skills we need to get along in the world.
Primates have a long period in which they depend on adults and during that time they acquire a huge number of skills which are not pre-wired but need to be learned from experience. It stands to reason, she says, that as human society becomes more complex and the knowledge required to live in society increases, the period of dependency will expand as well.
Ultimately, setting minimum ages appropriate for everyone is difficult. Neuroimaging technology has given us more information about the structure of the brain but behaviour is shaped by many influences, including experience, parenting, culture, social relationships and psychological wellbeing.
Mr Benson-Pope does not ask us to feel sorry for the law-makers though. The debate about the driving and drinking ages is really no different from that about getting tough on crime, he says.
"There's a cyclical frenzy about such issues that politicians exploit."
