9 x 19: It's the "Commerce Clause" that allows the feds to regulate firearms dealings.
And straight from the ATF:
(F6) Does a customer have to be a certain age to buy firearms or ammunition from a licensee?
Yes. Long guns and long gun ammunition may be sold only to persons 18 years of age or older. Sales of handguns and ammunition for handguns are
limited to persons 21 years of age and older.
Although some State and local ordinances have lower age requirements, dealers are bound by the
minimum age requirements established by the GCA. If State law or local ordinances establish a higher minimum age, the dealer must observe the
higher age requirement.
[18 U.S.C. 922(b)(1), 27 CFR 178.99(b)]
(F7) May a licensee sell interchangeable ammunition such as .22 cal. rimfire to a person less than 21 years old?
Yes, provided the buyer is 18 years of age or older, and the dealer is satisfied that it is for use in a rifle. If the ammunition is intended for use in a
handgun, the 21 year old minimum age requirement is applicable.
[18 U.S.C. 922(b)(1), 27 CFR 178.99(b)]
So eve if your state allows under 21s to buy guns, it will have ot be private sale and people would have ot ask why do you want this handgun?
You can always shoot your dad's, uncle's, or older friends handguns.
It is problematic. Maybe a local person, non-FFL could sell it to you. But it would be exposing them to risk and liability.
While your state law may allow you to possess or even own a handgun, most people are not willing to challenge Uncle Sam like that.
The Supremacy Clause of the US Constitution states that federal law is the law of the land.
And in light of the recent school shootings, I don't think you'd like to call attention to yourself like that.
So wait your time.
Get an AR-15, FAL, or a nice shotgun instead.