It bugs me now too. It’s been bugging me since I was drafted at 19 in 1966. If you’re old enough to defend freedom in a military uniform you’re old enough to do all that other stuff, period.
I’m a fan of the Heinlein model. If you’ve served your country, you can vote and do everything else. If not, well......
You can get an abortion at any age and without your parents permission. So if you can’t buy a gun until you are 25, then you can not get an abortion until 25 either