To be honest, that's about really all there is to it. Governments decided that at a certain age, you're considered an "adult". Other than that, I honestly don't see a factor that says someone is or isn't an adult. For example, most people would call me 'an adult', but really, I'm still a kid at heart; I love going to the playground with my daughter, going sledding with her in the winter, and I watch most of her favorite shows with her, and have no complaints about it. I can't say that's how I am when she's not around, but I know ALOT of people that would say those are childish actions. I'm the type of person that still tries to see the good in people, even when it is obviously not there, and I always hope for the best, even when logically, it can't/won't happen. I make mistakes that some people would say an adult should not make, yet here I am, making those mistakes anyways. On the other hand, I know few people at my age who have gone through the things I have; I lost my little sister several years ago, after being hit while riding her bike ~ she died in my arms at the hospital. I lived under a bridge for a month, up in NH right before winter, after the mother of my child and I broke up. I've gotten banned from a college campus (although I suppose that's not really an "adult" thing hehehe). I could go on and on, but I think that you get the point. I think it's an individual definition, but on a whole, can be generalized as someone who has reached a certain age, as defined by their country, who is supposed to be legally and morally responsible in said country, following their laws and customs.