It could be maybe cause Edward never had a first love at all, he is totally new to that. Plus i think i remember reading something, that he might had been changed ages ago but that he still is like a teenager, somehow. So yeah thats just two theories, allthough i consider the first one more.
Maybe because you know how he said he can read everyones minds except bellas...he probably fell in love with her for that so thats probably why he didn't like some old lady:) Hope that helps
Although he's been living for over a century, he's still physically 17 years old. He is more likely to be attracted to someone at around the same stage of maturity and development. The only beings who are as old as he is are fellow vampires, and most vampires would struggle to co-exist in a close relationship with a Cullen because of the human blood/animal blood issue. Also, the Cullens are trying to pretend to be human, and he knows that an apparent 17 year old dating a much older woman would attract attention to them.