I don't get it.. Is this the reason men in their twenties Don't have the ability to take responsibility for themselves and grow up...
When you look at how culture has shifted in the past 50 years one of the things that has been predominantly lost in our young men is the desire to grow up and be a man, some social commentators have called it the Peter Pan Syndrome.
It wasn't too far back in western culture that young men (teens) looked forward to the day that they would be considered a man within their community whereas now many don't want anything to do with what would be considered "adulthood" predominantly because of a nasty word known as responsibility.
Most "primitive" cultures have some form of a rite of passage that a young man goes through before he is considered a man by his community but western culture has lost that although some Christian communities have actually worked towards returning to some form of a rite of passage for their young men because with it seems to bring the expectation (and desire within the boys) of "growing up".
To answer the OP, IMHO, there is no age that determines being "grown up." I've known kids that were 13 or 14 that were "grown up" and I've known persons who were in their 60's that just