Sales from a dealer are regulated under Federal law, and apply in all states. Sales by a private citizen to another resident of the same state are NOT governed by Federal law (except you can't sell to someone you have cause to believe is ineligible to own a gun)
SOME states permit private purchase by 18 yr olds, some do not. Since you did not share with us what state you live in, we will just have to send out the bloodhounds, and track you down.
HOWEVER- even if you can legally buy a handgun in your state, you STILL cannot buy handgun ammo from a retailer until you are 21.
And welcome to the forum. Stop by the intro thread, say hello, and tell us about yourself.