He's older than Stephanie. Steph is a year older than Tim, but I know Cass was older than Tim by two years at least. And Bruce once took Cass to visit Jason's grave and said they were the same age. So basically, Cass and Jason are the same age...(probably 20 if not 21).
That's the problem with making Steph go to college. She has to be at least 18, but then that would make Tim 17, and the guy's wandering Europe on his own. Sure doesn't look 17 to me...hm.