strength
A post earlier from Rifleman1776 makes a good point, in that many grade schoolers 9-10-11, do not have the strength to handle a full size, or even youth size rifle from anything other than a support. Bamaboy, who was pretty wiry and active kid ( and is now taller and more able than his Daddy!) could not easliy manage a full size longgun till he was 11-12 yrs old. Till then he had to be content with a Cricket, and a bit of time on the 77/22. The sem- 10/22 was an option too, but he liked to make noise w/ a semi more than shoot well, so we had to limit its use!
At 11 we went to the Mini-Mauser .223 and shooting sticks.
I thought about pistol caliber carbines, and B-boy shot my Marlin .357, but single shot only, as the hammer and safely letting down same (no cross bolt on my rifles) was too tricky for him (or his Mom), requiring too much hand strength and understanding as a novice. Closely supervised, .38's in the Marlin were a nice step up, but the action was too much of a challenge.
He shot the Mini-Mauser .223 well, and graduated to a Rugerbolt 7.62x39 at age 13. This was a dandy youth combo and I recomend it to many, but there was not enough reach w/ the x39 for much of our ROW shooting, so .243 was next and he has shot same ever since. The x39 comes out now and again for woods hunts, but is largely a niche rifle for us now, despite my affection for it.