hmm...nail polish at age 6.
My mom said I ouldn't wear any makeup until I was 16, and even then, I was only allowed very neutral lipstick, a little bit of concealer, etc...
So at 12 I bought a bunch of makeup, and started weraing it anyway. I always work mascara, but applied it carefully so she would notice....by the time I was 13, it was coloured glosses, 14 was the eyeshadow and blush, and by 15, I was wearing a full-blown face of makeup. From black Cleopatra eyeliner, to bright red lipstick, anything went. and it sill does! Not sure how I managed to get away with it, although we did have lot of fights about it. (I almost always won...)
Of course, I also had pink hair when I was 15, so eyeliner didn't seem quite as serious, I suppose.