You and I will have to disagree on this. Twenty one is the legal age for alcohol which can be argued too. Also 21 to buy tobacco and 25 at most agencies to rent a car. Three years at this level (18-21) makes a big difference in maturity and an equally big difference in acting on impulse by a young person, especially when an action in harming another with a weapon is so final. BTW, I'm not anti-gun; I own a weapon.
We can agree to disagree. That’s fine. I’ll end with this.
The constitution doesn’t talk about alcohol (other than the poorly thought-out amendment), tobacco, or rent a cars.