I know a lot of people are saying age is just a number, but it depends on how mentally and emotionally mature the OP is, and how mentally and emotionally mature the lad is. One of my mate's sons is in a long term relationship, and even though he's 21 his world does tend to revolve around his XBox and football. His girlfriend is 18 and she's actually had quite a few rows about it. He's lucky that she's mature enough to voice her opinion and let him know stuff is annoying her, but when it happens the initial reaction from him is to take it as a personal attack as opposed to her just letting him know how she feels. At 18 and 21 there's a world of difference between boys and girls, in regards to what goes on in their heads. As they get older they start to tally up more, but at 18 the difference can be noticeable.
OP, if you want to go for it then do, but don't jump in with both feet. Spend time getting to know him and see if he's on the right level as you, and if he is then great, but expect to be making exceptions and compromises a lot with him.