Spike is a talking baby dragon, I don't see what's so strange about him hanging out with adults, teenagers, or kids. You have no evidence he's in his teenage years and there's far more evidence to the contrary. As I said, the show says it, as well as the merchandise and all related material. If you want to believe he's a teenager, go ahead, bro. I won't stop you. A lot of people, including myself, will disagree with you though.