when you commit adultery; cheating

An adult is when you're an, uh, well it's when you think you're a big boy, but really you're just a child in grown person's body and you hate your job and the other person that lives in your house.

A person who has stopped growing at both ends and is now growing in the
middle.

Adulting (v): to carry out one or more of the duties and responsibilities expected of fully developed individuals (paying off that credit card debt, settling beef without blasting social media, etc). Exclusively used by those who adult less than 50% of the time.

What Peter Pan told you not to be

Prejudice based on age, especially against young people.

Being a responsible adult. Used by immature 20-somethings who are proud of themselves for paying a bill.