Originally Posted by Doge

Because on long descents, sprints or attacks they need to keep up with the adults.
And you have juniors that hold 28-30 on a flat - or slight incline and a 1% decent >30.

Edit: An adult has a 53X11, the kids have a 52X14. The juniors are typically faster if they have the gear.

Below is from the 15-16 Nationals. As everyone was on 52X14, the playing field is equal. But when they race adults, it is not. Still - 54mph is difficult on 52X14