I haven't researched it since I was that age (I'm 50 now), but I was aware at that time that an 18 year old could OWN a handgun, but could not buy through an FFL until 21. Private sales were legal at 18, and if given the weapon by a direct relative or parent, ownership could be at ANY age.
However, I vaguely also recall that at some point before we got CCW passed in 1994, because open carry was all there was available to us,...some folks found it disturbing that some 16 and 17 year olds could and would be openly carrying, and the cops could do nothing about it. So Glendale was the first (despite state pre-emption law preventing them from legally going so) to pass a law prohibiting people under 18 from carrying. It got challenged immediately by somebody and was amended that anyone under 18 had to literally have a signed permission slip from their parents on a prescribed city form for the purpose. Relatively soon after that, someone at the state level introduced essentially the same law, and it passed,...unless I am remembering this all wrong. I'm at the age that is possible now.
All I know is that when I was 16, I bought my first handgun (a Dan Wesson Model 15) from an old duffer cowboy who figured if I was intelligent enough to discuss with him what and why I wanted, and had worked hard enough to afford it, that was old enough. Dad approved, although he wasn't present at the sale. It was just my first face-to-face gun transaction. Part of becoming an adult, as far as I'm concerned.