The age isn't the issue here, personally, I was 17 and my boyfriend was 21. In your case, the issue is the fact that you are two completely different ages in life. The fact that she is just finishing highschool, and you are looking through grad school can cause problems. She's going to get into college, meet many people, and she's going to start developing herself and finding out who she wants to be and the like. Likewise for you, while you may be focusing on your studies and looking towards the future, she may have her focuses elsewhere (depends on the person). All in all, those things, as little as it seems, can cause problems.
The choice is yours, I don't think the age is that big of a deal. But what your plans for the future are may NOT coordinate for her plans in the future... and if you're looking for a serious commitment, and she's just looking for a boyfriend, then again, there's going to be problems.
It's your call... weigh the scenarios, and think about what you want. At the end of the day, it's your choice to decide what you think is best and will suit your lifestyle and what you want.