There is a boys racer in the local paper, and dh has been wanting to get ds1 one for ages. He is 9 and about 136cm tall. The bike has 20inch wheels and a 17inch frame. I've googled and age 9-13 seems to be 24inch wheels, but I don't know if this is just for mountain bikes- maybe racers are different?
if he can sit on the seat nad feet touch the ground but not so he si straining to then it is the right size frame. when he pedals his knees want to me slightly bent not up to his chest so best take him for a test cycle before committing!
My 9yo dd (about 138cm ?) has a 24" bike which is just right. I'd look at the bike and take ds along; see how much upward adjustment is left on the seat post. But if it's going cheap it might be worth it anyway, even if he can't use it beyond the next yr or so. (Tbh sounds a bit small to me).
I have decided not to get it. Even though it is the local paper it is still quite far away, and I don't really want to drive that far for no reason. It does sound like it would be too small. Never mind