12 year old.

JAPANic said:

I couldn't wait for my son to grow any bigger so I went out and ordered him a Specialized Allez the other day.

It will be too big for him, but he is growing quickly.

In the meantime, we're going to lower the seat, put a short gooseneck on the stem, and small 160mm cranks.

He's going to struggle for a while changing gears, but I think he'll manage.
While he's starting out, no clip on shoes and we'll only ride along a cycling path down by the local river that has grass on either side for safe crashing.

He has over 9 weeks to prepare for his first race.... 10km around a lake down by Mt. Fuji. I've told him he can come last and go slow, just to take any pressure away.

My wife, myself, now my eldest son....one more son to go and we'll have a good little team....

Is 12 a good age to start?

As a father of five grown (none have been to jail) I suggest that 12 is ok if it is his choice and he decides how far is fun. If this is your thing he'll toss cycling when he goes teenage at 15 or 16. It needs to be his thing.
Good luck to you.

There are several small racing bikes with small components and 24" or 650C wheels, Felt, Avanti, Merida, Hillbrick all come to mind.