To act mature, to act as a 21 year old.
" Stop being immmture, you got to be 21 about it."
by jkk April 04, 2006
To own up to or acknowledge and accept responsibility for actions and/or wrongdoings regardless of the repercussions and opinions of others
" I already know you lying so Be 21 about it."
by URBANEDUCATEDREDNECK March 27, 2016