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Already there are 11 under-40 MPs: four in Labour, including its leader, four in National, two Greens and one in Act New Zealand.
On parties' recent polling averages up to late August that would double to at least 21 - more than one-sixth of Parliament.
Labour would on its recent-August poll average of 39.8% add one under-30 MP and five 30-somethings, bringing its under-40s total to 10.
National on its 41.6% recent-August poll average would get one new under-30 and four 30-somethings, for a net gain of four - one new 30-something replaces the departing Todd Barclay (27). Also, Chris Bishop (34) would be out unless, as he expects, he captures Hutt South off Labour.
So National's gain is three or four and new total six or seven.
The Greens get Chloe Swarbrick (23) on their recent-August poll average of 6.2% and, if they get to 6.5%, Golriz Ghahraman (35). So their under-40s total would be three or four.
New Zealand First has some younger candidates but not in winnable list places on its 8% recent-August poll average.
To the likely under-40s total add a rise in the number of 40-45s, which would take the total of true post-baby-boomers close to a third.
The numbers will change between now and election day. But it is clear there will be many more under-46s.
That's still a minority and those in their first term from 2017 to 2020 will have limited influence. But their weight will grow over time and in future terms.
If Labour heads the government in the 2017-20 term, most of its existing top team is under 45 and four are under 40.
So Parliament is now not far away from a majority of MPs well clear of the baby-boomers' long shadow. These people have had different life experiences, think differently and have different policy ambitions.
Fifty-somethings Bill English, Paula Bennett and Steven Joyce have a passing-era feel about them. That doesn't mean they won't head the next government. But it does mean the most they can do is tweak existing policy settings and fight the spreading political brushfires and wildfires.
Some of National's 2008-17 tweaks have been large, notably English's social investment. But the public response to Jacinda Ardern's elevation to potential prime minister indicates a growing wish for something different from even large tweaks.
Last week's exposure that public servants passed on Winston Peters' superannuation overpayments to ministers (and thereby to their political advisers whose responsibility is to their ministers' political and election needs, not to the wider public) has added to suspicions in some quarters that the public service is at risk of being politicised.
Fixing that will be one expectation of those who see in Ardern a beacon of real change.
But to what policy territory does that beacon point?
Different people want different change.
Under-40s, particularly under-30s, have known nothing but the post-1984 market-first economy and are disadvantaged by the large and growing distortions it has driven - in housing, in entrenched social inequalities, in a tax system heavily favouring the better off and in not dealing with pollution and private exploitation of resources.
Ardern's response has been a mix of social policy tweaking and a partial promise on tax which does envisage taxing income from capital gain (now almost entirely untouched) but leaves the decision whether to tax wealth/assets/land until after a working group has worked out how to do it.
But it is not just the young who want big change. Many - as a visit to a Gareth Morgan meeting shows - are middle-aged and even baby-boomers who think society and its subset, the economy, have gone wrong. Some want new settings. Others want to recover something they feel was lost in the 1980s.
This diversity of hopes was evident in northern democracies in votes for widely varying would-be saviours: Donald Trump and anti-immigrant parties, old-left Bernie Sanders (United States) and Jeremy Corbyn (Britain), protest parties like Italy's Five Star Movement, polling in the high 20s, formed by clown Beppo Grillo and macropersonalities Justin Trudeau (Canada) and Emmanuel Macron (France).
The parallels are loose. Social tensions are not as high here. But the challenge to future policymakers is more complex than change for change's sake. Managing those complex expectations would not be easy.
And can the 30s-45s make truly transformational change? Or will such change await the next cohort?
That question cannot be answered definitively until well into the 2020s. Meantime, Parliament will be different after September 23 and the difference might prove over time bigger than many expect.
*The numbers above are my count from age information I could collect, so may be subject to minor adjustment.
Colin James is a leading social and political commentator. ColinJames@synapsis.co.nz