I agree that the Bella/Edward relationship is rather immature. However, I didn't have a problem with it in the first books because Bella was just an immature teenage girl. In the 4th one though, I was disappointed that Bella's character never seems to really grow up. Even though she was married, it disturbed me that she was literally begging for sex. I also agree that the books are not the best for young/immature teenage girls. I'm an older teenage girl myself, and I don't think the books are totally horrible, I just found them a little shallow.
Setting that aside, if you're looking for a good Fantasy book appropriate for younger ages as well as older, try Fablehaven by Brandon Mull. It has a responsible, mature, teenage girl as a protagonist and the ideas are so fresh (I've been getting sick and tired of Fantasy books that are rip-offs of Harry Potter, LOTR or Star Wars). The Fablehaven series is good, clean fun. I just finished the third one and it is amazing!