When I was fifteen years old and read The Riddle-Master Trilogy by Patricia McKillip, I was taken with Deth and the Morgol's relationship, and thought it was one of the most mature romantic relationships I'd encountered in a book. Later on, I realized that I felt this way because it was a relationship where both parties rarely saw each other. She was ruling her land. He was always traveling in his role as the High One's harpist. It is mature, of course, that they weren't teenagers who wanted to be together 24/7, but still strange to me that one of my favorite romances was one I loved basically because they didn't spend a lot of time together. They had duties and roles and those took precedence over their love. Their love for each other does, in fact, become very important to the plot, but it is a low-key romance that co-exists with other relationships.
Another romance I liked as a teenager was Peter Parker and Mary Jane Watson, from the Spider-Man comics. That was interesting one because Peter was interested in Mary Jane physically from the moment they met, but dismissed her as too shallow to have a relationship. It wasn't until they'd both been through a lot of growing up that they became a couple.