People with oriental looks (me being one) generally look younger than they are, that might be the reason!?
Eh, I wouldn't agree with that personally. It is harder to judge their age but they don't exactly look younger. However, the 20 year old gymnast for China clearly looks the age. Even some of the girls listed at 16 or 17 look the part, but there are one or two and there is one in particular (can't recall her name) that looks extremely young.