i believe that if a relationship is not exploitative in ANY way (including financial, but especially emotional), and if it is between two consenting adults who are old enough to understand what they are doing, then it's ok. depends a lot on who the people are. in my opinion, i feel that most teenagers probably shouldn't be involved with anyone more than 10 years older than them. for me, i just wouldn't ever go there. 10 years is half of your life when you're 20, at 30 it starts to be different.
on another note, people in their mid-30s, if single, really have the best of all worlds, as far as age ranges available, 10, or more, years in either direction. it's a cool time to grow as a person, with a wide range of hot experiences.