At the Merriam-Webster web site under the topic of Words We’re Watching there is an article titled Adulting (The verb ‘adulting is all grown up). The current definition is:
“To ‘adult’ is to behave like an adult, specifically to do the things – often mundane – that an adult is expected to do.”
They note that current use for that word took off about a year ago. TIME magazine also discussed it on June 8, 2016 in an article by Katy Steinmetz titled This Is What ‘Adulting’ Means. Not everyone approved. At Cosmopolitan on June 20, 2016 Danielle Tulio ranted to Kindly Shut the Hell Up About “Adulting.”
In 2013 there was a book by Kelly Williams Brown titled Adulting: How to Become a Grown-up in 468 Easy(ish) Steps, which was discussed in a June 23, 2013 New York Times article by Aimee Lee Ball titled An Advice book by a 28-Year-Old? Not Quite.
But that Merriam-Webster article noted that back in 1980 adulting was instead used more specifically as a synonym for committing adultery.
A cropped and Photoshopped image of a couple walking into St. Johns College came from Wikimedia Commons.