The book's target audience is teenagers. The writing is incredibly bad, but it doesn't matter. It's in the first person and it is so detailed that the author succeeds in putting the young reader right in the story. If you allow yourself to get caught up in Stephenie Meyer's fantasy world, it works. She had an explanation for all of the twists she put into her vampire world, and quite frankly, it was refreshing. Obviously, True Blood thought enough of the twists to come up with their own hit.
I didn't have trouble with the age issue because everything about this relationship is forbidden, so what does a 100 years of age difference matter by comparison? It also works because Edward is a vampire so his human qualities have been suppressed, repressed and the vampire ones dominate. The book goes into that. In fact, the writer goes through great pains to compare their bodies to marble, stone, cold. This could be a metaphor for the way they're usually dominated by their vampire side, whereas their human side is repressed. So this is as much a new experience for Edward, as it is for Bella. It's not about him being a lecher, but about remembering how to be 17 and human again.
But, if you really found it hard to get into all that suspension of disbelief thing, you will love this review, about why Book 4 should not be made into a movie. I haven't laughed so hard in a long time. Lots of spoilers:
http://latimesblogs.latimes.com/entertainmentnewsbuzz/2008/12/twilight-10-rea.html