A 4 year difference is not very large nor out of the ordinary. That is the year range for schooling. Did you never hang out with people older than yourself? It could have been a group thing, and he could have been friends with someone there or they also could have become friends prior.

I interact with people twice my age. Even "hang out" on occasion. Four years is very small.

And the dude had the correct response to a 16 year old's crush. "Ew, you're a baby" not only works to keep her cognizant of age disparities, but also gives her important perspective about how normal people older than her will perceive her.

Judging from the rest of her post, she has come to understand that lesson in full which means the older dude was a respectable party -- at least, in that situation.