I'm currently 17 years old and in a couple of months, I'll be 18.
People should not be ashamed of being whatever their age is and still watching Avatar. I'm a senior in HS that's done a presentation in front of a class full of kids about Avatar in SPANISH. XD I've also gone to a scholarship interview for college and they were just asking me random questions and one happened to be "What are your favorite T.V. shows?" I did not lie and told them Avatar: The Last Airbender because the rest of T.V. is fairly cliched and depressing. (IMO) And you know what, one of the interview ladies who could've been in her late 20s goes, "OH! I watch that too!"
So there you have it. You're never too old to watch Avatar.