The law does not ban possession by 18-20 year old persons.
Maybe not possession, but you can’t BUY one, right? That means a straw purchase, which is an end-around the intent of the law.
Why?
I think kids as young as 0 should be allowed to own a gun, and kids who take courses should be allowed to use a gun, even in self-defense. My grandmother used to take her rifle to school to fend off mountain lions in Niles Canyon, CA. Everybody had one, and learned to get along somehow.