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.
a 120cc dirt bike is suitable for a 17 year old
For a four-year-old child, a bike with 12-inch wheels is typically recommended. This size is suitable for children around that age and provides a good balance of stability and maneuverability.
A very small bike
The recommended girls bike size for a 10-year-old is typically a 24-inch wheel diameter.
The recommended bike size for a 7-year-old child is typically a 20-inch wheel diameter.
The recommended boys bike size for a 10-year-old child is typically a 24-inch wheel diameter.
For a 6-year-old, you should consider getting a bike with a wheel size of 16 inches. This size is typically suitable for children around that age and height.
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 10-year-old girl, a bike with a wheel size of 24 inches is typically recommended. This size is suitable for children around 9 to 12 years old, depending on their height and inseam measurement. It is important to consider the child's comfort and safety when selecting the bike size.
For a 12-year-old, a bike with a wheel size of 24 inches is typically recommended. This size is suitable for most children in this age group and provides a comfortable and safe riding experience.
The appropriate bike size for a 12-year-old is typically determined by their height. A 12-year-old child with an average height of around 4 feet 11 inches would generally require a bike with a wheel size of 24 inches. It is important to ensure that the child can comfortably reach the handlebars and pedals while sitting on the bike.