DD started at 11/6th grade.
I figured she was old enough to get blemishes, then she was old enough to be able to cover them up to feel more confident. We started out slow, but by the end of the year she wore what she wanted. I still guide her choices and emphasize that makeup is to hide flaws/accentuate your good points and should be as natural as possible. I had one Bobbie Brown book and bought the one for teens. She really emphasizes a natural look. DD used those to get started.
Before that we had her brows done professionally, but just to clean them up - she inherited her dad's full brows. Again, she was old enough for it to bother her, so we felt it was time to do something.
Luckily, she also inherited thick, dark lashes so she only uses liner/mascara on the bottom lid - you would think she applied it on the top lid as well. She really likes the dark look on her eyes - that's her "style". I just make sure it isn't too bad before she goes out the door.
ETA: the only down side: she was recently asked for ID at a restaurant that offered college discounts!!! It isn't just the makeup, people have thought she was older for some time (she takes dance classes with high school girls and they thought she was their age!), but the makeup doesn't help!

