Should the legal age to buy a gun be raised to 21?

I've been watching as some people are now arguing that the age to purchase a gun should be 21 because they feel people under 21 are not mature enough to own or handle a gun. If we do that, shouldn't we also raise the voting age back up to 21 since those under 21 have been deemed to be too immature? Should we also change the law prohibiting those under 21 from entering the military or hold public office due to maturity issues? I notice that nobody comes out and claims that the thousands of young men and women who were drafted or enlisted, and were killed in Viet Nam, and other wars, including the one we are in today, are too immature to be in the military.