My favorite is .243 Win. My mom passed her Rem model 600 down to my twin daughters for deer hunting when they were twelve. It’s a fast, flat shootin’ cartridge with plenty of punch for mule ears and won’t knock the young ones off their feet. Ammo is easy to come by with a variety of factory loads for everything from varmits to deer sized game game. I even take it on long carry hunts because of the lightweight package. I believe most rifle manufacturers offer youth sizes in this wonderful and versitle cartridge as well as full size arms.