In terms of most games, it's entirely up to the host. Some are strictly 18+, some let people under 18 play if deemed repsonsible enough,
I started out watching, learning, take pictures. Then, one day they asked me to play. That was over a year ago, and i haven't had a problem. It's all about who runs the game, and your attitude towards the sport.
But i was very lucky, and an exception. From what I've seen since, 18+ is a nice, nice rule.