I am 22 and I am still a HP fan
Some of my friends are older and are also fond of reading the books and visiting Internet sites, etc.
And - well - I <i> don't </i> think that HP is a children's book, not PoA and GoF, definitely! I am really annoyed when people smile or shake their heads when they see what I am reading!
However I think that you tend to be less "obsessed" when you are older...