As long as you speak/behave/play the game in a mature manner, you're not a kid.
If you want a specific age as an answer, then I would have to say you're an adult after you graduate high school.
doesn't matter the age, I have met many adult children. it's all about a fun attitude and willingness to be an unselfish team player, if you can do that, your light years ahead of most people who play this game.