I was 7. It was as a gift for my 8th birthday (the next day). Truthfully, I think I was too young and not responsible enough to deal with what I had to do. On top of it, we found out I am allergic to fake metals including the so called hypo-allergenic metal. This meant that I could only wear 14 carat real gold. At the age I was at, I was losing a lot of earrings or the backs, and it was a nightmare. As I grew older, I was still allergic to most metals and could only wear "cheap" earrings for 24 hrs or my ears would get red, be painful. At some point back in my 30s, I got fed up and took my earrings out. I let the holes close and never looked back.
I don't mind parents deciding to have their kids' ears peirced. I don't even mind it that they get their babies ears peirced. As I said, I think it's an individual parents' decision. But, I do think you have to pick and choose when to do it. The "Tween" years is NOT the time to do it unless you have an uber-responsible kid. Either do it when they are babies, or, wait until they are a teenager. That's my take on this.