Gramm is correct. You probably weren't planning a pro career anyway, but plenty of people take up boxing after their 20's.

I was 57 last year when I started for the first time. And technically you could still fight amateur until 35 here in the US (and likely similar in the UK etc) without having to go to Masters category (older boxers.)

But really, MOST of the people in our gym don't "fight" in competition, they train, they may shadow spar (no or very light contact), or a few will actually spar occasionally. Most of those who spar are among the smaller number who are planning to compete.

You can enjoy boxing with or without competing.

As to your son, he is old enough as long as you can find a gym that supports him -- probably with other young boxers. AND especially as long as he ENJOYS it.