Green’s Dictionary of Slang

(US black) adult, mature, grown-up.

1992		Eble Campus Sl. Apr. 5: grown – behaving overly mature. ‘Susan, don’t you be too grown with Adam tonight.’.
1994		N. McCall Makes Me Wanna Holler (1995) 375: I’m a grown-assed man.
2005		B. Coleman Rakim Told Me 101: ‘I didn’t want to just walk up on these grown-ass people and demand shit’.