My first gun at the age of 11 was am Ithaca 37 in 20 ga. I shot thousands of rounds through that and strongly encourage anyone to have child learn a pump gun for the discipline of making the first shot count and the coordination.
My small-framed, 13 y/o nephew has shot the Remington 870 20 ga. like a champ since he was 11. Last summer he said he felt it wasn't fair to compare our shooting when I was shooting a "12 ga. cannon", so I bought a used Benelli M2 in 20 ga. and carried that all last fall instead.
The M2 is the softest shooting gun I've ever handled and is an absolute joy to hunt with. The prior owner had the stock cut down an inch for his daughter, but had it done so the extra inch could be put back in for an adult or larger youth. What a great option.
Even when I wasn't hunting with Ben this fall, I started taking the M2 with me to hunt roosters, ducks and even geese. It was phenomenal what you can do with a 3" load of 5 shot and an extended choke tube these days. It killed geese just as dead as any 12 ga. I can remember. I can't really say there's any reason a youth starting out today can't shoot a 20 ga. with 3" shells at just about anything for as long as they want.
If Ben is still as ethusiastic and keeps improving, he might earn the M2 as a 16th or 18th birthday present if i can bear to part with it.
I cherish my experience shooting a pump gun for the first 6-8 years before I ever got to shoot an auto.