If deer are the object of the hunt, a .243 or .257 Roberts should work fine. For bigger stuff, though, I would go for the .260 or the 7MM-08. Bigger bullets and better penetration. A 12 year old should not be shooting at ranges beyond the effective range of either of these calibers. Don't make the mistake of getting a really lightweight rifle. As nice as these are for a young shooter to carry, a medium weight gun will be steadier and kick less at the moment of truth.