Always! I don't think I've ever not written fanfiction for a show, movie, or book I've really enjoyed. There are some amazingly written ones out there, and it's a great place for young writers to practise and develop their skills. Plus the community aspect of writing fanfiction is a lot of fun!
As a teenager I wrote a few fan fics for Star Trek Voyager and Charmed, but I rarely read fan fic because one, I don't like reading lots of text on a screen, and two, a lot of them featured character pairings I did not like.