The old truth was that you should have 1-2" clearance between your crotch and the top tube, but current designs often use sloping top tubes whch messes up the old reference. There are plenty of sites that'll give you some guidelines, do a search for "bike fitting" and see what you'll come up with. Try a couple of the different methods and them pick a comparison from the middle of the group. The most crucial value is that when you're in the saddle you should be able to have your leg almost fully extended when the pedal is at its lowest position.
The best bike to buy for a nine year old active boy is a 24in-wheel bike with a 3-speed hub gear.
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.
A very small bike
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.
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.
If that is the correct size for her feet then that is fine.
Yeah if they have the right size dirt bike and gear
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.
a 120cc dirt bike is suitable for a 17 year old
I'm a 14 year old 5 foot 6 inch girl and I use a 26'' bike. For a 5 foot 10 inch boy I would reccomend a 28'' bike, so there's plenty of room to grow into.
between 150cc and 250cc
It depends on the size of the 11-year old!