My parents' rule was that I could do whatever I wanted with my hair, but nothing permanent to my body until I was 18.
That's my rule, too, pretty much. I don't care what my son does to his hair. For Pete's sake, it's only hair. It's not worth a fight. Wise parents pick their arguments. My son's hair has been long, shaved, blonde, black, green, blue, etc. It's only hair.

He also has his ear pierced. He has for several years. Sometimes he wears an earring and sometimes he doesn't. Ear piercing is so mainstream for both sexes now that I let him do that.

But, that's it. No more piercing and no tattoos until he's grown. I have nothing against it, per se. I do personally think it's not attractive to have 78 tattoos and 59 piercings, but to each his/her own. I have two tattoos. However, I got them when I was about 30, and I finally knew what I wanted them to be. Perish the thought that I would have gotten a tattoo when I was eighteen. It would have probably said, "I love Jon Bon Jovi" or something equally stupid, and I'd be sick about getting it now!