I'm only 19 and I want someone my own age, I tried to tell him that I'm not interested in a relationship with him and that I want to just stay friends but he still flirts with me whenever we meet. I don't want to hurt his feelings but I really don't want someone so much older than me, what the hell do I do?
that's his problem that he can't get over you. you already told him you're not interested. If things get too far out of hand or he begins to creep into your comfort zone, just stop hanging out wiht him. In the mean time, just be firm that you really just like him as a friend. Persistence is key. He'll come around eventually to know that nothing will happen between the two you
You have to hurt his feelings whether you like it or not. Tell him straight out "you flirting with me is making me very uncomfortable. I like you as a friend, that is as far as it will ever go. Please stop flirting with me or we can't be friends anymore."
There are a lot of older guys who are simply obnoxious and rude about this sort of thing because, unfortunately, some younger guys aren't very assertive and if they are persistent enough, some guys will give in. So yes, as Revan said, you need to be really, REALLY clear, perhaps something like "I enjoy spending time with you as a friend, but I'm not attracted to you and never will be. So I need for you to understand that and stop flirting with me. Otherwise, it's just going to ruin the friendship entirely." There's no way around it... and, sadly, there's also the possibility that once you make it clear you aren't interested except as a friend, you might discover he's no longer interested in being friends.
^ With the others. You have to give him the cold shoulder, you need him to see that you're not interested back, also don't bother being friendly, be straightforward and serious with him. Otherwise if he persists to keep on flirting, then just cut contact with this guy.