When someone of an age that would imply they had some level of maturity, (ex. 25 and up) says/does something completely juvenile.
Someone out of college age who should have their lives together but doesn't, that critisizes others of a younger age (18-23ish) for being immature or "highschool-ish", when THEY are the one whose life is going nowhere.
Brought on by the stupid childlike behavior of people who should be mature adults, yet choose to act like idiots. Used mostly in South Texas.
I know Mark is 26 years old, but he is such a little kid. Mark would be considered, 'so 28'.