Men under age 25 cannot rent a car, and not by law, but rather by the insurance industry's hard statistics about testosterone-fueled behavior for men under 25. Those young men also shouldn't be able to buy assault weapons, and for the same reason.
I was an incel from age 12 to nearly 22, so I know how depressing that can be. Worst years of my life. Then my outlook on life changed dramatically for the better. Raising the age for men to buy an AR-15 clearly would save lives.
Furthermore, on their 18th birthday, their criminal record is automatically clean no matter how many times they've been locked up in juvenile detention. Think of that as the juvie loophole to legally buy the weapons for mass murdering little children.