Oh, we had body piercings too.
It's not a crime, they won't go to jail and in the case of my ex-drug using daughter who pierced her entire face...lol....she grew up and got rid of all of her piercings so she could work and because she outgrew the novelty. By age seventeen, we can't really tell them what to do...they will just do them at eighteen. I'm more concerned with bad behavior and I think the more we resist, the more they are convinced we are hopelessly uncool. Here's a funny story.
When 37 was ten, my daughter wanted her ears pierced. Not being a big carer about this, I told her, sure. When we got there, she chickened out. So what does 37 do? He boldly said, "I want MY ear pierced" in an arrogant way...you all get it. Immeditally I told him sure and he had it done, almost crying when his ear was pierced. Scott, who was still part of our family, strangely also wanted his ear pierced so we went home with two boys with a pierced ear. Two weeks later the holes had closed up. I never said anything, but obviously neither wanted a pierced ear. They just wanted to shock me. 37 even said, "Wow, the boys at school are shocked that you let me do it."
I didn't say anything. Sometimes you don't need to. Neither has ever had a pierced ear again.
I'm very strict about not breaking the law. But when it comes to their body, their hair color, etc., well, I just don't get involved. My oldest daughter was a Cosmetologist before becoming a Pastry Chef and the girls in Cosmetology would always do each others hair in weird colors. On July 4th one year, Daughter came home with red, white and blue hair. She wanted to die my hair lavandar. I told her no thank you...lol. Her hair is no longer colored at all. The tatooes are where they can't be seen. She has two.
Both girls are doing well. It's all good to me

When it comes to behavioral boundaries, things that put each other in harm's way, I am very strict. But I feel it is not personal if my grown kids do something to their bodies that I personally don't like. I don't even think it's bad. It's part of expressing independence and growing up to me. My biggest easy child has a lovely tatoo. It is high on her arm where it is almost always covered up, but it says "Family is Everything." It made me cry. She means it. I love her tatoo. by the way, that particular tatoo we did fund. It was my daughter's graduation present. Wow, it's expensive!