My son is 13 now but he started with a 6mm rem and i shortly after he took his first deer bought a rem. 700 youth in 243. he took several with it and now seems to have taken a liken to my 25-06.
I have no complaints with the 243 as a matter of fact he took one this year with it shooting a speer 100 gr round nose. dropped a doe in her tracks at about 125 yards.
i like the little youth model so much i am going to put it on a lamianted stock and add a floorplate. it is a tack driving little gun.
But i wouldnt have had any objections to a 260rem or 7mm -08. i just got a great deal on the 243.
the biggest thing i would recommend is to let them shoot alot with the rifle you end up with.
i believe you can't practice enough.