Starting aged 45, on the other hand, is an "extremely stupid" thing to do, Dr Clive Dreyer says.
However, the Dunedin Hospital emergency department doctor did not let that stop him, joining son William in learning the martial art.
Now, five years later, the pair both have gained black belts.
"It's more a sport for younger people.
"It's a high-kicking sport with most of the kicks above waist height and I had all the flexibility of a plank," Dr Dreyer (49), of Dowling Bay, near Aramoana, said.
William (11), who attends John McGlashan College, said while his father might not have kicked very high to start with, he "looked good to me now".
They had worked hard to prepare for their black belt, which they achieved in Christchurch earlier this month.
Unable to sit his black belt until he was 16, William instead achieved his junior black belt.
"He's probably one of the youngest in New Zealand," Dr Dreyer said.
It was a gruelling process which required months of extra training for Dr Dreyer, while William continued with his normal training.
"He's been very supportive.
"It's our special thing," Dr Dreyer said.
The idea behind learning the martial art was for William to learn to protect himself.
"We tried it together so we could help the family in case anything happened," William, the oldest of three children, said.
The best thing about learning taekwondo was doing it with his father, he said.
They were also learning to play the guitar together.
