I may of course be wrong, but I don't think there is, by law, a minimum age you have to be to get a piercing (apart from ones on under 16's that could be considered to be "intimate sexual contact", such as a nipple piercing). As long as a person is mature enough to understand fully what they're doing, then I think it is legal. However, the actual places that do the piercings usually do have a minimum age, usually 18, I think so that it makes things simpler and less likely to have parents getting involved afterwards etc
If I am wrong, please correct me, I was told this a while ago but they could have been wrong themselves!