I don't prescribe to the thought of having a rifle a youngster can grow with as far as competition. I understand a lot depends on the shooters size/strength, but when/if they start out small and light, they should be allowed to upgrade as they grow. An example might be going from a CZ scout, to a CZ 452 Military Trainer, and then on into the serious rifles such as Anschutz and the like. By the time a youngster gets ready to outgrow the Military Trainer- he/she may well decide to venture into another arena such as High Power, Service Rifle, etc.
Rimfire competition can be fiercly competitive and quite costly, so I hate to see a kid sent out under-gunned. Whatever he/she starts out with should have, or be able to be set up with peep sights.
But I degress. Choosing a rifle also greatly depends on what type of competition they are gearing up for- Benchrest, 3-Position, Silhouette, Running Target. Maybe from 8/13 years old one rifle may do all for him/her. But if they are serious- they should look to specialize by 14/16 years. But, that's just my opinion.