At 7 or 8 years old, after learning on a .22, I put my kids on an AR chambered for 6.8 Spc. It has hardly any recoil. When he turned 12, I gave my son a Browning Micro-Midas in .308. I handload, so I was able to load a 110 gr Barnes pretty light for low recoil. It's worked out great. If you don't handload you could buy managed recoil ammo. .308, 7/08, .243 and plenty more are all good rounds as long as you use light loads. Nothing will screw up their shooting long-term like putting them on too much gun too soon. Same goes for hand guns.