When you go to a convention at a younger age, you don't think twice about how old any of the chicks are. Even if you're eighteen, you are still at the border. If you confuse a fourteen year old girl for being sixteen or older and someone tells you, you think "damn, she's hot for a fourteen year old". You're kind of allowed to make that mistake.
When I went to Anime Boston, I hadn't gone to an anime convention since I was eighteen or nineteen. Now I was twenty-two years old, and it made a huge difference. A lot of times it was surprisingly easy to tell the younger-than-sixteen girls apart, and being at a campus for a while you develop an eye for characteristics. It becomes easier to identify the girls that are eighteen and up, and those are the ones you keep your focus on. After all, most all of the others are six years younger than you at the least.
The eyes cannot always be trusted, though. At one point I ended up talking to a girl who looked, spoke and acted as if she were about twenty years old. I mean, even her posture and body language was mature. I know it sounds silly to say such a thing, but when you see a bunch of high school girls at the mall after looking at girls on a college campus for five years, you can definitely note some different behaviors.
The older I get, the worse it becomes trying to identify age groups. Now girls that are in College seem too young depending on the age. I'm having trouble distinguishing 25 and 30. I look for wedding rings more often than cleavage. It's an insane time, and it sucks when you hit up a convention and have no way of telling how old anyone is.