Why do underage girls keep flirting with me?

I'm 22 and I never talk in school or church. Anyway, I'm one of the usher for my church along with my other 4 brother, and teenage girls keep looking back at me a lot, they even try staring me down. At the store one time, this 16 year old-ish girl was bending over to put something in the cart, then she look up and froze a while with her draw drop looking at me, she then smile. I had a omg look with a confuse puppy eyebrow since her parent was next to her. Another time, these private school girl in uniform was visiting my University for some reason, and I was sitting by the big water fountain thinking, before I know it, these girl were taking picture of the fountain right on the other side of me, and then took picture of her friend too with me in the back ground. Why are they doing this? Are they trying to get me in trouble or make me look like a child molester? Your thought would be much appreciated.