My DD13 started wearing makeup when she started Middle School- age 11. That first year she would wear it occasionally and lightly. By last year 7th grade (age 12) she was wearing it daily.
For me this is kind of like when to allow leg shaving...When my DD reached a point that she was going to feel self-conscience to not wear make up- I let her.
I don't know a another 13 year old that doesn't wear it and my DD has large and varied circles of friends (school friends, ballet friends, theater friends, church friends).
Leg shaving, that's another thing I'd let her do now but DBF thinks she's too young. I'm thinking of giving her a razor in her Christmas stocking this year.
What she wants to wear IMO is fine. My DD would wear that in 7/8th grade if she wanted, however at that age she didn't really wear the eye stuff everyday but would if she was going anywhere. Beginning in 9th grade is when she really started wearing the eye stuff everyday. As a senior she still mostly concentrates on the eyes and lips. I think at 13 she is old enough to wear eye and lip stuff but please not the heavy eyeliner I see on some young girls, that to me is what makes them look cheap/trashy.
If she waited to 16 she would be a junior in high school and really be the odd girl to the other kids if she didn't wear any make-up.
I'm not making her wait until 16, not at all. I do think she can start playing with it, wearing mascara and lip gloss is a good place to start for me. I did find out that DD21 did wear the heavy eyeliner, she thought it looked good on her, she still does wear it that way, she still thinks it looks good. LOL I'm more of a natural type, I wear skin tones, use Bare Minerals and wear mascara on special occasions, I don't wear it every day because I'm "just" at work and our offices don't get foot traffic.
My DD is 13 and has shown no interest in make-up at all

. She has mentioned that some of her friends wear it and if she wanted to I would let her, but I think she doesn't want anything else to do in the morning. She isn't very self conscious about her appearance, some days I'm lucky if she brushes her hair before she runs out the door.
LOL DD was just like this up until this year.......she asked me for a hair straightener the other day, that way it will be "easier to fix her hair every day" LOL
My mom let me start wearing it in middle school. She taught me how to use it correctly, though. Nothing heavy, nothing obvious, nothing trashy. I don't think 13 is too young at all (I was 13 and in high school!) as long as she knows how to properly apply makeup. I know that some salons in our area do a "teen's first facial and makeup application" kind of thing. Could you call around and see if anyplace will show her how to do a LIGHT and APPROPRIATE makeup?
I don't think any of our stores do this but maybe I'll do as a PP suggested and take her to Macy's to the Mac counter or something.
I have all boys, so take my opinion for what it's worth. But I feel it isn't about age as it is about appropriateness. It's more important where they are wearing make-up (maybe weekends only to begin?) and how they apply it. If a girl/woman has no clue how to put make-up on, it looks worse on them (even at age 16, 18, 21+) than younger girls (11, 12, 13) who do apply the right amount correctly.
This is how I feel too, I told her that for now she can wear it on weekends and around the house to practice and get comfortable with it, then next year when she's starting High School we can talk more about what she can wear to school. I started wearing make up around 14 and I remember one of my first tries I went to school and people laughed at me because my blush was too dark. I remember washing it off in the bathroom and having to scrub my face with those rough brown paper towels, well that made my cheeks even redder than if I'd just left the blush on! LOL So I talked to Mom that night and she and my older sister gave me a lesson and I practiced for a while before trying again. LOL You've all basically said how I feel but I do appreciate all the replies and suggestions. Thanks!!

I do think 13 is still a bit young, but I'll stick with my weekends and at home deal.
