The boy's height would be more relevant than his age. The basic requirement is that when he straddles the bike he should have an inch or two of clearance between his crotch and the top tube of the frame. But what you really need to consider is what he wants to do with the bike. A BMX is more of a piece of sports equipment than a bike. If you want to take your kid on rides with you a BMX isn't particularly well suited, they're meant to be ridden very energetically out of the saddle during fairly short bursts. Many think of BMX bikes as being suitable kids bikes because their overall size is small, but that's pretty far from the truth. Usually they come with 170-180 mm cranks for instance, and that is the same length as for full-sized adlut road bikes for instance. Same goes for the handlebar. If your kid, when he's in the saddle ends up with the handlebar at about face height it's far too high for him to be able to ride effectively. Anyway it's not like you have a lot to choose from, as 20" bikes have a solid domination over the market.
if you want a motorbike at the age of 12 your best size would be an 80cc, 100cc, 125cc but if you have not had experiance get a fun bike
Depends on what size he is i would say a 20 inch bmx bike.
The size that fits. Without knowing how tall your 11-year-old is, it's hard to say.
18 to 20 inch. Bike
53
a 120cc dirt bike is suitable for a 17 year old
A very small bike
Yeah if they have the right size dirt bike and gear
A 125 two stroke or 250 four stroke would be good power wise. Just make sure the bike fits for their physical size.
It depends on the size of the 11-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.
Depending on what make you get, either a 100, or 125.......
between 150cc and 250cc
Yamaha XT250
sure
In the UK - 49cc or less.
Kawasaki Kx500, any year will do, but the best bike for a beginner is a Yamaha yz490