Introduce a little at a time. Maybe at 11 lipgloss and 12 blush and 13 eyeshadow - something along those lines. :)
Both my daughters started wearing a little bit of make-up at 11 or 12 and I decided to see what would happen if I made makeup a subject of partnership for us -- as if makeup was OUR thing together. It really worked because when they realized I wasn't going to be an adversary about makeup, they felt able to ask me for advice, etc. Eventually, that allowed me to say, "That's too much eyeliner for school," or whatever. Nowadays, I get my best makeup tips from my 18 year old, who is always on the forefront of good mascaras!
My daughters (I have three) started wearing lip gloss at 11. At 12 and 13 they wore eyeshadow and mascara. At 14 and 15 then came the eyeliner. They are now 13, 15 and 16. None of them are heavy handed. I never had to say it was too much. I think that the "natural" bronzers and eyewear help to keep things at a minimum. Coupled with the fact that too much is more for the "goth" look. There are days that they do not wear any make up and they never do blush. I have noticed with each one that they each realized wearing make up was not all they thought it would be cracked up to be. There was the extra time in the morning to put it on, thus having to get up earlier. Then there was the taking off at night, hassle! Once they were given the permission to wear makeup it seemed once they got it out of their system to wear it the thrill of it was gone for them. However, the attention from boys comes not with the start of wearing make up but with the start of wearing a bra! That's a whole other story!!!
the one thing I can say on this is if they want to wear it they will , with my now 18 year old the grand age came at 11 when she wanted to wear makeup , i was so against it nad said no. then one day i wnet to pick her up form school early as I had forgotten about a dentist apointment for her and when she came out of class she had the full gear on. that noight we discussed how this was not safe to borrow others make-up (very easy way to pass pink eye and so on) so we compromised on getting her own set that she had to pay for it, I met with the parents of her closest friends and we all came to agreement on what the kids could care with them ( that way even if they did share they all had the same stuff)i have not always agreed with her choices in the way she applied it but i got over it and so did her father.
My now eleven year could care less which makes it so much easier but i know the day will come and it will be addressed then.
Just remember that they want to immulate their friends so just keep a good eye on who they associate with and get to know their parents , try to have a talk with their mothers and see what rules they apply in their house and see if you all can agree on something it is much easier when the rule is universal.
This to me is an area that many have lost in recent times because kids dont really only have friends in their own neighborhood where all the parents know each other and talk . It is up to us as parents to reach out and make sure we know the families our kids are with. I usually try to have at least one party a year where i invite the parents of my kids friends over so even the kids see that we talk.