I will let both of my kids get their ears pierced at some point in time if they really want to. I want my kids to be old enough to take care of them mostly by themselves so I probably would not go much younger than 8/9 or so. If they ask for pierced ears I think it is something that I will make them wait 4-6 months after they ask just to make sure it is not something they are asking for on a whim. After that if they still really want them then I think I will allow it. Although only one set of holes until they are teens. No other piercings or tatoos, etc. until they are 18.
I think I tend to be fairly liberal on this issue. I think of it kind of like a picking my battles kind of thing. In the grand scheme ear piercings just don't seem worth the battle to me.