Something I realized along the way :
It's not that I ever was any good at telling a girl's age by her face, but when I was 14, I knew what 14 year olds acted like, how they talked, what little differences there were in dress vs an 18 year old. When I was 18 I knew what 18 year old girls acted like which made them easy to pick out from a crowd of teenage girls. Once removed from that age group my old references are obsolete. Little differences in fashion, in hair style, in mannerisms, in language - a variety of things that you aren't privy to once you move on to the next stage in life.