When I was eighteen, I dated a thirty-two year old. That was not the best decision that I had ever made in my life. He seemed nice, and sweet and like we had things in common and originally, he even lied to me about his age. (He told me he was 27, when I was younger I held pretty fast to a policy that eight years was the largest gap that was acceptable when it came to dating. And, well, ya know, what was just one extra year between friends.) We started dating and it quickly became obvious that things were not going to work out, because we really were in different places.
Almost immediately, the talk of marriage started, and let's be honest here, at eighteen, getting married was the very last thing on my mind. I wanted to go out, have a good time, club, and all that kind of thing. And, he wasn't against it, but he also wanted me to seriously think about marrying him. And kids. He wanted them, baddly. Now, I don't know that I will EVER want children, but even know, three years later, I still know I'm not ready to have children. I need to finish college and feel secure in my own life before I'm ready to do something like that.
The fact is, dating a guy that much older than you means that there will probably be major differences in what you two are looking for in the here and now. Sure, there are exceptions to the rule, but you should probably try to make sure that you two are REALLY in the same place, first.