I find it hard to believe that your parents are okay with this. You're a child. He's an adult. No matter how long you've known him, unless he has the mentality of a child, there's no way that you two could have anything in common. Also, it's morally wrong, and if your parents aren't okay with it (if they are then they need serious help), it's illegal as well.
So no, I don't think it's okay. If a 20 year old came after my 15 year old daughter he'd be in jail so fast he wouldn't know what hit him.
You're too young to know what's good for you. That's why parents are supposed to step in and protect you from predators. The fact that you say that your parents are okay with this, and have been since you were 13, and he was 18, is just sick. So no, I don't think this is okay, no matter what the circumstances are. It's wrong on every level.