Instead of lecturing others to act "like adults", why don't we stress the need for maturity? Adulthood does not by its own beget maturity. In Ancient Rome, the Paterfamilias did not declare his son to be a man until he demonstrated by his conduct that he was willing to take care of himself and behave like a free man in civilized society. A male could have been fifteen, twenty, or even thirty years old before his father would accord him the dignity of being a man.
Mature behavior comes in all shapes and sizes, regardless of the age of the person who steps up to the standard.
Mature people know that they do not have to force circumstances around them to get what they need. They do not have to lie, cheat, steal, manipulate to make things work out.
Mature people accept that they cannot have everything they want it . . .at least right away.