Yeah if they have the right size dirt bike and gear
between 150cc and 250cc
A very small bike
If he needs a regular bike, I would get him a full size 26 inch bike. He is at the point where he is about to start his adolescent growth spurt.
I would perfer a Honda CRF 50 there a very great 50cc training bike
The bike that you should think of buying an active 9 year old is a 24-inch wheel bike.
Try a Yamaha TTR 125 or CRF 150 (not the motocross version).
It depends on the size of the 11-year old!
a 120cc dirt bike is suitable for a 17 year old
For a 13 year old it depends on your size and your experience with dirt bikes, but an 85 is usually the best size bike and fastest for a 13 year old. Unless you are really tall which you would need a 125.
It all depends on how big he is. If he is small for his age he will need a smaller bike or if he is big a larger one. Best thing to do is ask the attendent at the bike shop.
The bike that you should buy for your nine year old son is that with a 20-inch wheel with suspension forks and adequate front suspension.