My first question was, "Grown-up or adult?" and my second was, "If grad students who study this sort of thing can't answer it, why are you asking me?"
Anyway, my answer was, "A grown-up is someone who assesses risks and rewards before taking action, and acts to minimize the risks and maximize the rewards, while doing as little harm to others as possible."
So, for example, it's acceptably grown-up to put your best foot forward when applying for and interviewing for a job, even though it's almost certain that if you are hired, someone else will be harmed by not being hired. It's not acceptable or grown-up to actively sabotage another candidate. By my definition.
However, I don't study these things, and so I'm probably wrong, and so I would be interested in how you would define a grown-up.