Unfortunately, the problem with maturity is that it's impossible to affix a value to it. The best thing we've got is age, and I think we can all agree that it's very far from perfect.
Something that bugs me, though. Well, if a teenager isn't prepared for a relationship (sexual or emotional) with someone of an older age, what exactly makes them so prepared for someone their own age? We've all heard plenty of rape and abuse stories, seduction and emotional destruction, all those things with people that are under 18. I'm not sure if I would go so far as to say the 17-year-old is more likely to be abusive in some way than the 34-year-old, but when a quick googling turns up statistics like, "40% of teenage girls ages 14-17 report knowing someone their age who has been hit or beaten by a boyfriend. (Children Now/Kaiser Permanente Poll, December 1995.)" (granted, I'm not entirely sure how valid it is) you can't help but wonder. Teenagers are certainly just as capable of being abusive as older people.
And to top it off with an anecdote, my best friend's dad is married to a former student of his. They've got a picture up on the fridge of him playing guitar to a classroom full of 5th graders with her in the picture somewhere. I believe that they entered a relationship when she was in her 20s or so and they only found out later and coincidentally that she used to be a student. I don't think he's much more than 10 years older than her, but they're in a very good, loving relationship and the example stands as a testament to what's possible.