I'd say write the story you've got in your head/heart and let the story be exactly as you feel it should be. Categorisation can come later
My mom is one of those adults and she is 48. According to her many of her friends read them too. I believe that because they are easy to read they attract casual readers. With books like Twilight there could be going on same things that are behind 40 year old Justin Bieber fans.