I never have to show ID when I buy ammo...I would stop going to that store
I don't get carded for beer anymore either but I'm pretty sure if I sent my ten year old son in any store to get beer or shotgun shells there might be a problem.
If Internet dealers are asking for proof of age it's safe to assume they have already been hit with some type of citation/fine or whatever because a minor bought ammo from them.
Get a C&R , mail it to them and end of problem