So nether of these would do ?
I would think it would depend somewhat on the youths experience and body mass.
For a young person unfamiliar with guns I would opt for a 410 or a 28 or 20 gauge single shot. I know you indicated that a 410 is not something you would consider. If you are talking about a older, larger youth, I would agree.
For a smaller lighter person I would disagree.
A 410 is as capable of taking Grouse as any other shotgun. They all have the same range and penetration potential. The only real difference is in the amount of lead each calibre sends downrange.
However, larger volumes of pellets do offer an advantage so if the youth is capable of handling the larger bore shotguns, a larger bore would make sense.
My opinion is that a 16 year old who weighs over 150 lbs is probably quiet capable of handling a 12 gauge.
For a 90lb youth I would recommend nothing bigger then a 20 gauge. And for a 60 lb youth, nothing bigger then a 410.
That's not based on any rule or principal. It's simply the way I do things.
In other words, you know your child better then I do, you can decide whats best far better then I can.