Grown Down 

Command. The opposite of "grow up." It serves as an instruction to the listener to cease activities that are considered by the speaker to be overly refined, mature, or stuffy. Often used by people who wish to continue to employ high school jokes about how "gay" someone is.

Friend 1: I don't find it funny when you call people "gay" as a way to say you don't like them.

Friend 2: Grown down.

Grown Down by Learned Hand 2 February 2, 2010

grow down 

When young kids think their grown. And it's obvious that their not!

Mom: I can't believe you just said that.
Daughter: Well mom, I guess I'm gettin old....
Mom : grow down, No one says that any more

grow down by Sherulz2 June 21, 2015