That was the question on a local radio station's morning drive last week. Answers varied widely. Coming home with a first paycheck, paying a first rent check, signing off on a house, bringing a baby home, were examples. Some set the sights lower, such as remembering to pick up napkins with the food at the fast food joint like their mother always had (if that's what it takes, I'm still waiting...).
I'm not sure where to draw the line for me. Getting up by myself in the morning without nagging? Buying my own clothes/doing my own laundry? Shutting off lightbulbs? Doing my own housecleaning? Buying furniture that you don't intend to sleep on? Refraining from spending money when I had it?
Of course being an adult isn't the same as being recognized as one. Little things like being treated as a houseguest rather than as a kid when you visit your parents. Getting addressed with honorifics like "Mr". Not getting carded when buying cigarettes or alcohol. Getting your own credit card solicitations.
Having survived twoscore and more years, I figure I must have made it to adulthood at some point, sense of humor and taste notwithstanding. I'm not in quite the same hurry to reach the next milestone (millstone?), when I start getting senior citizen discounts.