I don't think using the term has to do with not knowing how to be an adult, but rather not wanting to do the responsible thing you know needs to get done.
Going to the PTA meeting? Ugh, no thank you.
Standing on line at the DMV? I'd rather get concussed.
Getting a colonoscopy? How about I just take a nap?
Uh, they use it when whining about paying bills...
Or when taking care of their own kids. They brag about how well they are doing at "adulting" when they feed their kids 3 meals a day...