The girl of my friends startet at 13 to put on the heaviest makeup, like in Boudoir photography or in artsy videos. Extra long lashes, heavy colour, eye shadow, foundation, masks, eyeliners, Lipstick, concealer, rouge..you name it. Her face turned from kid to something from a fairy tale where she had vibes of a very attractive young adult with some childish giggle added. Two years later that eroded and now she is almost makeup free and her outfits occasionally show some colour blocking. Thing is that from observation I can tell that they start often at 'go wild' and erode from there rather the other way round.

I think they need to be a 'teen', and it really should start small and then progressively get into whatever they want. If its something they are really passionate about, maybe I'd drop it a little earlier but I'd prefer they'd use mine or my wifes face to practice on first.

13 but not full face.
16 for full face or special occasions.
18 go wild!