I'm going to preface this by saying that the opinions expressed below are just that - opinions. Not judgments on the choices of others. Many of you no doubt handle this differently. This is how we handle the issue of piercings at my house.
Ears - my girl got hers done when she was 12. She asked before then, but has always been a little young for her calendar age maturity-wise, so when she asked I told her we were going to wait awhile and explained about taking care of piercings, infections, and so on.
As for other piercings, when she is an older teen (15+) I'll be open to negotiation on some piercings (more ear, nose perhaps, belly button perhaps), but only after a frank conversation about risks, care, maintenance, and what social statements she might be making, intentionally or unintentionally.
Tongue I will not allow her as a minor, and I'll explain to her why. Or better yet, I'll let my ER doc friend talk to her about tooth damage and nerve injuries and what happens if you get a tongue piercing infected. Knowing him, he'll probably even be able to produce some really disturbing photos that should take care of quelling the urge. Same for genital and nipple piercings. Considering that my child does not tolerate pain (a paper cut still induces tears at 12), I would be really surprised if any of these piercings ever appeal to her.
Also, even for her initial ear piercings, I took her to a reputable piercing studio.
As for my son, if he ever gets interested in piercings, I'm going to have a battle on my hands. Not with him - with my spouse. He feels very strongly that males shouldn't have piercings, although when pressed, he has a hard time saying why he feels that way. I feel very strongly that by the time my son is a teen, the same restrictions, or lack of restrictions, on piercings should apply to him that apply to his sister. For now, we've agreed to disagree, but if the issue does come up, the discussion could be, um, interesting.