What do you think about me responding to a guy that's older than me?
Why do you plan to respond to him?
I find him pretty annoying, but my friends all think he's attractive.
It's fine that you want to chat with someone that's older, just make sure that the age gap won't put either of you in any legal trouble. If you're underage and the guy is an adult, then you both could get into a lot of trouble for chatting. Be considerate and avoid talking to people that you shouldn't.
He's not that much older then me.
It seems that you are more concerned about what your friends think then what you want. You should only respond to the guy if you truly want to be involved  with him. This will help you to avoid an unwanted situation.