When, if ever, do people reach a point in their life that they can definitively say that they have reached adulthood? Also, how do you define adulthood?
I think of adulthood in three different stages.
Stage 1: The generic adult. When you start acting like an adult. I've known 14 year olds more adult than 29 year olds. You're not completely on your own, but you're not acting a fool and mooching and sitting around like a teenage loafer. (I miss those days.) You're paying for your own luxuries and at least some necessities, but you still need the adults you grew up with and depend on them to at least some extent. This is where I think a lot of people stay.. for a long while.
Stage 2: The parent. Even if you don't have kids.. this is when you take on the role of the adults you used to rely heavily on. You have your own place, and while you might need some sparing helping here and there, you're now the one doing the helping. Mentoring others, getting them jobs, raising kids, and teaching teenagers and young ones the things you wish you knew. Maybe your parents got sick, or the relationship changed, but the parents don't feel like parents anymore--they still command respect and honor, but they lack the authoritative touch they had before. They've passed on all the information they can. I don't feel like most people get here (without kids forcing them here) until at least their 30's.
Stage 3: You're past the age where all the adults you knew growing up are functioning. Either they died, or they're on their way there. Even if you needed their help by now, they couldn't offer it to you.. and there's probably young people depending on you in some way, shape, or form. Whether it's your own kids, or the young people you're managing at work. You're the authoritative figure whether you really want to be or not. The things you grew up with are no longer recognizable. The current young generation has forgotten about them, and without active effort you will lose connection with the current generation and notice a significant gap between the two..
Those are my generic breakdowns that I think about. They don't come with age, people have jumped straight to that old person stage 3 thing at very, very early ages.. but in general, I think you become an adult at teenager level, and that's why you're expected to act like an adult by your 20s. Because you should've already been acting like one for quite some time now to ramp yourself up into that adulthood area.