Most Helpful Guy
Well, it's important to remember that 18 year-old guys are morons. Even if they're brilliant, and thoughtful, and sweet, they're still morons. And the reason that they're morons is that they don't know that they're morons. When I was 18, I was certain that I was an adult and that I had life figured out. Now, in reality, I was heading down an academic/career path that was making me miserable and depressed, I was convinced that I could only get along with people who shared my social and cultural views, and the second a girl I had a crush on showed any interest in me, I started planning out our whole life together.
Five years difference means different things at different points in life. My girlfriend is actually five years younger than I am, and even at the mid-twenties/early-thirties point, it still shows sometimes. It showed even more two years ago when we started dating, and I can only imagine what it would have been like if it had been when I was 23. That's a pretty huge gap.
That being said, you shouldn't be deciding things for him. If you're communicating to him what you want out of the relationship and you're not getting those things, then you shouldn't be together. But if you're communicating what you want out of the relationship and he's indicating that it's what he wants as well, he's the only one that can make that determination. It's his job to decide what's best for him, and your job to decide what's best for you. If those things match up, then great. And if they don't, then that's when it's time for the relationship to end.