In England and Wales, ear piercing is legal at any age with parental permission, but most of the parents I know through daughter's school said "not till you're 10-11" and got it done during the summer holidays, to give it time to heal before the start of term (as the primary school didn't allow earrings). My nephews all had theirs done as babies though. My daughter (nearly 12) isn't interested as yet.

Daughter's secondary school (11-16) doesn't allow jewellery (or retainers) in piercings other than one small stud per ear, and I think that's quite common.