I’m surprised you don’t know that you can buy a long gun like an AR-15 or a light machine gun from an unlicensed dealer at any age. Seems like something the son of a US Marine would know. Have a look:
It is common practice to set legal ages for activities that require maturity such as voting, driving, and drinking…lawcenter.giffords.org
And if anyone has a known agenda that negatively influences their views on sexual assault, I’d avoid accusing women like Professor Ford and take a look in the mirror instead.