i've always wondered about this because so many young models do not look young at all...they look very mature for being an early teen. how does one truly know what their ages are without looking at a birth certificate? especially if they're foreign?
^about younger teens looking older, sometimes that just depends on an individual's looks. Daul Kim was only sixteen when she started but often looked like she could pass for twentysomething, mainly because of her bone structure and strong features, and also because of the very adult air of self-possession and confidence she had in photographs.
I guess the only thing that will tell the actual truth about a model's age is her passport (which any high-fashion model is bound to have, since they travel so much between fashion weeks and to shoots etc).