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.
Yeah if they have the right size dirt bike and gear
between 150cc and 250cc
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
A very small bike
Try a Yamaha TTR 125 or CRF 150 (not the motocross version).
24" is a wheel size, not really a bike size. Not all 24" bikes are the same size, and not all 9-year olds are the same size. Basic rule of bike fitting is that there should be 2" or more of clearance between the crotch of the rider and the top tube when straddling the bike. Then the saddle should be at such a height that the rider's leg is almost fully extended with the foot on the pedal in it's lowest position. Any bike that allows for this has a chance of being a decent fit.
The bike that you should think of buying an active 9 year old is a 24-inch wheel bike.
a 120cc dirt bike is suitable for a 17 year old
A bike for a 4 year old normally requires stabilizers, unless at least he or she is able to ride solo. A typical bike that suits this purpose is one with a mountain bike (MTB) frame.
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 depends on the size of the 11-year old!