I started to really experiment with makeup around the 6th grade, and my mom did not really let me wear a ton until after I turned 12, then she tried to set limits that were obviously NOT adhered to. I was doing full-face bronzer, dark purple lipstick...basically if I liked it, I wore it. I loved, and still do love, gold and silver eyeshadows. Occasionally, my mum would buy me a Lancome shadow or two but it was just cheap stuff until about a year and a half ago when I started the big upgrade.
I have no problem with girls starting to wear makeup around the time they get into middle school as long as they are given lessons on how to apply it properly and that they aren't doing smokey eyes and vampy red lips. If parents are letting their kids go off with buttloads of makeup at the age of 10, I do have problems with it. Clear lipgloss only is what I think, that and the Smackers lip balms. A lot of people I know say 16 at the earliest, but I think when you are younger you can get away with doing more eccentric, weird looks that you might not do when you are a couple years older, and everyone SHOULD have the opportunity to try some out.
Crap, I just totally contradicted myself.