Ok, well let me start by saying that I'm 20 and I absolutely love this show to bits. That's right, I'm a mature(ish), socially active, employed, complete with a life, friends, dates and family adult - and I love this cartoon.
It's a shame that many people judge a show before they've actually taken the time to watch it. A cartoon can quickly and wrongly be labelled as a 'kids show,' and that just isn't fair.
Avatar is wonderful. The plot is consistently good and moves along at a nice speed, the animation is fantastic and the characters are well-developed and easy to relate to which really gets the viewer behind their quest.
Read the basic plot and yes, I'll admit, it doesn't sound terribly original what with the slightly reluctant hero, the strong female presence (whoop Katara!) and the comic relief dude who is always the one to get hit by something. But I promise watch an episode and you will be enlightend.
Avatar just makes me feel happy. It's like watching a Disney film - instant child-like glee :D Such is the way with Avatar. It takes you through a whirlwind of emotions but in the end you're smiling and wanting to watch the next episode.
So give it a shot and enjoy yourself. Age is not an issue with this wonderful little show :) (Oh and if you do find you really like it, you could always grab yourself a cheap multi-regional DVD player and get the American Season 1 boxset. It's a better deal in the end and will most likely take forever to be released in the UK.)