Thank you Yidnar, for at least wanting to discuss this.... You are a caring human being and father of many kids, who you wouldn't want to see killed in this manner.... Which is honorable, and can be easily understood by anyone, who is not in this thread for simply a partisan, political fight.
It is constitutional for states to put an age requirement for purchasing guns, including long arms....
States already do it.....as examp!e, Florida made the age to buy guns of any kind at 21 years old, in 2018.
Age to Purchase a Firearm
Since 2018 it has been illegal for anyone under the age of 21 to purchase a firearm. Florida Statute §790.065(13) states that a person under 21 years of age may not purchase a firearm. This applies to both handguns and long guns. However, it is not illegal for a person 18 – 21 years old to own a firearm. Although they cannot lawfully purchase a firearm, anyone over 18 years of age who is not otherwise disqualified an own and possess a handgun or rifle.
So, it can be done at the State level, but I'm uncertain if it could be done on a federal level?