Maybe no one ever does, and everyone else is just better at pretending than I am!
To me it would be understanding politics, having a clean enough house for unexpected visitors, knowing how to make myself look smart and sophisticated, eating and sleeping at sensible times, regularly.
But maybe, there are adults who do all those things and still don't feel like an adult, and think the things I have done are more adulty!
Or maybe we are actually all fully adult, and part of being fully adult is wondering if everyone else is more adult than you are....