For the youngsters, the rules of the mountain bikes are almost the same except few extra considerations. The top tube clearance and the reach are very important in this regard. The wheel diameter for these kinds of bikes is usually kept between 12 to 26 inches. The child must have access to the ground while seated for control and safety. The brakes must also be soft and work very well. Moreover, it would help if you preferred twist grip-type gears in these bikes.