If you want to raise the age for gun purchases, you need to also raise the age for voting. If a citizen is to young to come to the aid of state / society with their firearm (militia), then they are too young to participate in the governance of the state / society. They both go hand in hand.
“If you want to raise the age for gun purchases, you need to also raise the age for voting.”
...and joining the military. If we can’t trust 18-yr olds to own guns...we can’t trust them with a gun in the military.