Better grooming, photo-shop, cheaper more easily available beauty stuff, celebrity 'you can be beautiful too' culture, plus young people are aware of their looks much earlier than in the past i think. Hence very well groomed 13/14/15 year olds.
I started high school in '84 (12 years old) and wore long white socks and a awful massive pleated skirt, way too big for me, no make up, greasy hair and thought nothing of it. All the girls in my year was the same. No real awareness of appearance. We were all still playing out on our bikes with the boys after school and getting mucky. I remember looking at the girls in their last year in - gasp - thin tights and short skirts!
By my 3rd year i had started to take control of what i was wearing, to an extent, the skirt fitted and was shorter. The white socks had gone, the shoes were less geeky, and i was using a bit of mascara and lippy and washing my hair every day. I got it bleached when i was 14 and was the first one with 'coloured hair' in my year.
But some girls these days are still in primary school looking like mini celebs. Hair coloured, fancy cut, make up, clothes like adults, heels, nails - the works!