Edward Ferrars is definitely an underdeveloped character--I think he just doesn't have enough face time in the novel. I have never been of the opinion that Elinor and Brandon should have necessarily been best together, though. Underneath his quiet exterior, Brandon seems to be a true romantic, with a passion for music and poetry that makes him a better match for Marianne than the more pragmatic Elinor. And in the book, it's another year or after the main events before they get married, so she's not quite so young. But there again, part of the problem is the underdevelopment of that ending. Austen just wasn't as interested in the romances in her stories as she usually was in the comedy and social commentary, so she tended to leave a lot out, particularly as concerning secondary characters.
I suppose I'm not as prejudiced against large age gaps because I married a man almost 15 years older than I am. Whatever our struggles as a married couple have been, very few of them have had anything to do with our age difference. That doesn't mean I don't know that we're close to the outside of what can be done successfully, but I'm inclined to believe our dear author when she tells us that they were happy together.
