When I was over 18 and under 21 in Florida, I legally purchased a GP100 via private sale.
There is no form 4473 for ammo purchases, as far as I know. So, I am not sure if "straw purchases" would apply in that scenario, but I had friends or relatives who would acquire ammo for me, then, if I could not buy it myself.
(Edit: I'm in my 40s now, and still think that status offenses are a load of crap. If an 18 year old is old enough to sign contracts, and be charged as an adult for breaking laws, then it seems weird to say "You are being charged as an adult for being a minor..." Then again, some states charge kids as young as 12 as adults, depending on the crimes charged. Ok, rant mode off.)