Do u believe an AR-15 or any semi-automatic weapon should be sold to an 18 year old child ?

Because when you are 18, your brain is not fully developed. The actual age of maturity is around 35. Sure you can fight in a war but I often ask myself should we allow a teen to fight in a war.
21 seems to be a more fitting age for this as it should be if you ask me. I mean you have to be that old to actually buy a drink in the US.
Do you believe that a person who is 18 years of age should be allowed to buy a semi-automatic
weapon.