I started liking Harry potter in like 3rd grade, when I didn't fully understand what I was reading (I'm now a Junior in High School). That's about 8 years. I started liking twilight in 9th grade and and stopped in 10th grade. It's a huge difference.
Also, Harry Potter's plot is so much deeper with many more things going on. Twilight....not so much. I had liked Twilight at the time, because it was a cute and sort of complicated love story. Harry Potter just has more appeal for all ages and I can continue to like it through the years.
Plus...Twilight has just gotten WAY to commercialized to actually like. Sure Harry potter has some of the merchandise but they are doing it more in a subtle.
Plus there is that one simple fact; Harry potter is written WAY better (: