It's amazing how much maturing happens in the 17-18 year. One goes from a gun-toting danger to society to a responsible airsoft player once they hit the magic age of 18 /sarcasm. Ok I realize it is against ASC rules and the law to buy a gun when underaged, but I think the OP's sheer horror at a 17 year old owning an airsoft gun out of proportion. Yes he is technically a minor... by one fucking year. When someone turns 18 there isnt a magic switch that gets flipped and all of a sudden that person is responsible. Firearms safety comes from the individual using the gun, if you had so much concern maybe you could have gone over proper firearms responsiblilty with your 'friend'. 18 is to look good (and legal) on paper, not to make sure someone is responsible. I am not saying he should have been sold the gun, but don't overreact and bash someone who will be part of the community in a year's time.
As for the seller not ID'ing the buyer, as Brian said the buyer made a false claim. And no you are not always asked for ID when buying things that should require it. You should be, but the reality is people forget, are too lazy or too trusting. I've bought a .22 air rifle from Crappy Tire without showing any ID, or when I first turned 19 and went to a bar no one carded me. ID and 'legal age' of 18 is simply a way for the public to save face if bad stuff happens, don't ever think it actully means something important.