The part about being happy that Rob's not under age (well, the whole paragraph) - I had to laugh so hard! But I have to admit I'm glad too. A 17 or 18 year old wouldn't have appealed to me.I also always pictured Bella and Edward as older...maybe because I didn't want to feel so icky day dreaming abotu a 17 year old. (I cannot tell you how happy I am that Robert is not underage...makes it feel ok for me!)
Books turned into movies are a critical thing. It doesn't always work out well. While I love Lord of the Rings and I am pretty happy with the Harry Potter movies, there are others that were just bad. Like Message in a Bottle. Has anyone read the book? Back then I loved it and cried a lot but the movie was so boring that I considered leaving the cinema - they simply ruined a beautiful and tragic love story completely. But I'm not that anxious about Twilight. Of course they have changed thins and of course there will be things I won't like, but I think as long as they capture the magic of their love I'l be fairly happy.
Getting married young: I've got the impression (and please, please, please correct me if I'm wrong) that in the US it is not that unusual to marry before the age of 25 . Where I'm from that's rather unsual (at least among the kind people I know), so that's why 18 seems so young to me.