Expanded thoughts: The concept of "adulthood" is fundamentally about expanded responsibility.
Responsibility for the self, first and foremost. Additional responsibility comes with many "adult" things, but all of these are optional. Still, the more responsibility you take for things (particularly your own behavior or the effects of your behavior) the more adult-like people will view you.
What people don't always get with adulthood is expanded power, and failure to acquire sufficient power to match your responsibilities is the root of much suffering, both physical and psychological.
Sometimes, of course, that power isn't in our ability to acquire, and in my view that usually results from either tragedy or injustice. But sometimes it comes from decisions to take on too much responsibility without sufficient planning.
(See also: burnout)
People seem to think that being an adult means doing jobs you don't like. I would actually argue the opposite. Being an adult means you can quit a job you don't like.
Granted, it also means you can starve to death if you do, but still. Choices!