Fist off, I'm a senior and an 18 yo as well.

People keep on talking about how the relationship is potentially unhealthy for the 14 yo. It can also be healthy! Making mistakes and learning from them is important in life, if this relationship turns out to be a mistake for her, then maybe next time she won't date a guy who's "too old" for her (I don't necessarily think OP's too old). Thus learning from her mistake. Of course it could be horrible, but with the OP sounding like a nice guy, I doubt that his girlfriend will have a bad experience with him.

After thinking about how some of you people think that liking and dating a 14 yo means there's something wrong with you, I disagree. It does feel on the edge of not being okay, but for me I think the environment in which he started to like her is completely fine. I'm guessing OP met her and talked with her (his girlfriend) through hanging out with his friend (his girlfriends sister), not by trying to prey upon her (or anything silly like that (maybe silly wasn't the right word? replace it with terrible?)).