Jay is right. It depends on the kid's size. My 13 year old nephew is 6' and weighs 160 lbs. He has no problem shooting a 12 ga pump with 3" mags.(usually shoots it better than me, too) My wife is 5'3" and won't shoot a 12 ga unless it's a semi auto. She didn't even like the recoil from my 20 ga. Ask around and see if you can find a buddy that will let you borrow a 20 or a 12 semi auto and let the kid decide. Just my opinion......Scott
My first shotgun was a 20 ga pump, and if I still had it, I'd probably still use it. I was a skinny pup, and didn't have much of anything to absorb recoil. A couple years later, my granddaddy moved me to a 12ga after a day in the Pamlico Sound, trying to knock down greater scaup.
That's the rub of a 20, at least for duck hunting. The reduced recoil is great for youngsters. But with steel shot, your hunter will need a lot of patience and a lot of teaching to take good, close shots. Even with lead, the 20 let a lot of big ducks get away. Never used mine with steel, but certainly shot enough from my 12 ga to know there's a real big difference in killing power.
Other than that, like the other guys said... it really depends on the kid. A stout, strong kid can handle the 12 a lot better than a smaller young'un. There may be a tradeoff in the blind, though, and that's important to consider. You want the new hunter to have some success, not give up in frustration.
I started off with a 20 at the age of 12 and I think it helps to become a better shot.I still have this gun and if I have a spell of missing alot I will use this gun on a hunting trip and it will help with my consentration and fundamentals.By the way this is a colt 20 gauge pump bought brand new in 1968 my first gun,my second gun came about 3 years later and it is a 870 wingmaster that I have used up untill 2 years ago when I became a proud owner of my Dads Beretta AL-2.Love this gun,love all my guns .
If he/she can handle the 12 gauge with confidence and safety for waterfowl Go for it.The best thing is to hook up with someone who might have a youth 20 and a 12 with a shorten stock and do some shooting.My oldest just turned 13 and he has really surprised me with how well he has moved up to the 12 gauge,and we are somewhat vertically challenged in our family.
I do think that starting with the 20 is a good thing ,seems to build confidence quicker.
My daughter is 12 and shoots a 12 Ga 2 3/4 over and under no problem. My son now 9yrs 60 pounds wet, shoots a 20 GA Youth Rossi, and it knocks him pretty good. I would go with an auto 20. Barrel length is also critical on youth models. I have over 250.00 in 20 Gauges now. If I were to do it again it would be a 20 Gauge Youth Auto. A 3inch 20 holds the same amount of shot as a 2 3/4 12 GA.