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.
I would say a bicycle for preteens. But I'm not sure what sizes there are could you tell me?
I would buy the next size up from his pants size. If he wears a 12 I would get a 14
buy one from a shop huggies size 6 should fit if he/she is skinny
Shopping for discounted winter clothes, buy electric or ordinary bicycle, buy and install some solar applications to my house.
For a 14 year old boyfriend, you can buy him a mountain bicycle, to mark your one year anniversary.
small size
You should buy one that meets your requirements and budget.
A bicycle for two is a called a tandem. Simply type " buy tandem bicycle" + your Town or state into your preferred search Engine and see what comes up.
Check the sticker on the drivers door jamb, it will list the size it was built with. That is the size you should buy.
9
The same way you buy anything else. Do a bit of research, decide which one you want (size gotta match, you have to afford it, there has to be one available) then pay for it and it's yours.
It depends on size and gender. For example buy a male pitbull ,and since it depends on size,it should cost around six hundred.
You can buy them over the 'net, from any number of sources. Or a well-stocked bicycle shop is likely to have some in store.