actually, the mountain bike he tried and liked was also a 24 ( Trek 220). It seemed his size. The bike shop was telling me that the X24 is also a 24 inch bike but it is intended for a tall person that wants a BMX bike. My son wants a BMX but as he sat on one he realized he has to stand and pedal. He didn't like that part of it. But on an X24 he was sitting down. The bike shop also recommended the 24 inch one.