I see no problem with seeing 13 yo girls walking around in makeup. as long as it looks good and isn't layered on.
when i was about 10 my mom started making my face up for special occassions, because she knew how bad my face made me feel; and she'd go through all the layers. then i started doing my occasion make up when i was 12, and started wearing sparkley (pretty much invisible) e/s at 13.
even now, my version of a full face is an all day lipstick, liquid e/l, POWish e/s (light inner corners, and attention grabbing outer corners(i'm still real light on color though, nothing bright or darker than medium brown)) and loose powder.
i'll do foundation, loose powder, e/l, smokey type eyes, and lipstick or gloss for outings though...
i figure as long as you're not made up and in baggy sweats and tee shirts, or not made up (to some extent) and in a fancy dress, its ok. as long as the make up goes with the style and isn't overkill