I could have ghostwritten this post haha, we're the same age and I'm in the same situation. Always mistaken for a high schooler but yet get attention from older men more than younger. And I'm pretty shy/non-confrontational as well so I find it hard! I can relate!

To deal with it, if the situation allows it (i.e if I'm not at college or somewhere where my real age is obvious), I just tell them I'm fifteen! That gets rid of them pretty quickly (age of consent here is 16)! If they're a bit more forward, just looking them in the eyes and saying slightly condescendingly "I'm far too young for you, don't you think?" tends to put them on the spot.

As for the question of "why", guys our age, I feel, tend to assume that more conservatively-dressed girls would be harder to approach/less receptive to advances (this is a generalisation obviously, people all have different preferences, just what I've observed)? I've noticed that when I'm dressed more "on-trend" I tend to get more attention from similarly-aged boys than I do when I'm dressed in my own "style", which is more cute/youthful. Idk. Just my two cents worth.