So if a 17 year old walks into a tattoo parlor and piercing studio and says "I want to get my nose pierced", she should be turned away because of an arbitrary age? Why is 18 so magical? A 17 year old can consent just fine and know exactly what they are getting into. Bodily integrity goes both ways. I won't force DD to get any sort of modification, but I wouldn't deny it either because it is in fact her body and her choice, whether she's 7, 17, or 57.
Sometimes, you just have to pick a line, and stand on it.
Why is the driving age 16 in so many places? While it's obvious that most 6 yo are neither physically capable nor emotionally mature enough to handle the responsibility of driving, there are probably plenty of 15 yo who would do just fine. On the other end of things, most 26 yo are both tall and strong enough to handle a car as well as emotionally mature enough to be responsible, but there are also 17 yo who are not really ready to start driving. So why 16? The answer really is plain and simply that you have to pick something.
No there isn't anything particularly magical about 18. There isn't some huge difference between a 17 yo an 18 yo and a 19yo. A number needed to be picked. A line needed to be drawn.
Except we aren't talking about driving a car, or anything that requires certain abilities to be done safely. We are talking about piercings and tattoos. Why should the government even have that line? There is nothing about it that affect the rest of society. Having a nose pierced doesn't means someone can do something stupid and kill a whole bunch of people. It means they have a hole in the nose that isn't put there naturally. Heck, why have an older age for piercings and tattoos than for driving? Why not make the age 80 across the board, that way only those with extensive life experience will get these privileges.