I honestly don't think it depends on the person's age but the person themselves. I am 13 and starting reading the books when I was 10 and haven't lost interest at all but gained interest. Gained so much that I now consider myself an....obsesser.....but I do have a life outstide of the books. It all depends on the book and what you like. My mom knows a guy she's known for a long time and when they were my age he loved LOTR and he's in his 40s now and still loves them. So there's proof that it doesn't depend on your age at all in some cases.