I keep seeing people arguing the gun purchasing age should be 18 because the military lets 18 year olds have weapons...
Yeah. It's a dumb argument. So debunking said dumb argument is equally dumb (or at least somewhat meaningless).
The better argument is that the right to keep and bear arms is an enumerated right in our constitution, and that by default the full rights of an adult come to each of us when we legally become adults. It's the point at which we are legally allowed to basically do what we want, and are legally responsible for what we do. It's the point at which we can enter into contracts on our own (which includes a whole host of things, like owning bank accounts, property, enter into employment without parental consent, etc), be compelled to testify in court, gain the right to vote, etc.
That age of majority is set nationally at 18. So by default, the age at which legal adults should have the right to keep and bear arms (on their own, as opposed to under parental supervision) should also occur at the age of 18. This is not the same as when we allow someone to purchase alcohol, or buy cigarettes, or whatever. Those are not rights.
What I do find amusing is the slippery slope argument being made, pretty much unabashedly. So because we previously restricted the purchase of handguns to the age of 21, against the wishes of the gun rights activists of the day, we should now have no reason not to raise the age for rifles to 21 as well. In fact, I've seen dozens of folks making the argument that it's somehow "bizarre" that we require people to wait until 21 to buy a handgun, but they can go buy the dreaded AR-15 at the wee age of 18. It's such a travesty! Never mind that handguns are actually used to commit crime vastly more often than rifles, of course.
Great. Let's lower the age for handguns back to 18, where it should have been all along. There's no evidence that raising it ever reduced any crime committed with handguns anyway, so why keep a failed law on the book? The idea that we will now make hay out of the difference in age is astounding. Yup. It's a dumb distinction. It should never have passed in the first place. Using that age difference to push for increasing the age for rifles is just bizarre. And again, let's recall that the default for obtaining adult rights is when you become an adult. We really ought to have an extremely good reason for restricting it by age.
And no: Some people might abuse the right isn't a good reason to restrict it for everyone.