The time after the pointless teen years in which a young person develops what are called "standards" and "morals." It is a time in which eyes are opened and "The Truth" suddenly becomes clear.
Usually beginning when one enters college, said young person reads deep literature, searches for the meaning of life, and listens to college radio, all the while secretly becoming even more confused than when he/she was an lame 16 year old.
The Post-Adolescent Idealistic Phase usually entails naive 18-20something year olds upholding hopeful, yet unrealistic morals. Often, utopian dreams can be confused for "The Truth."
After the phase has ended, the young person must beware becoming highly hypocritical and/or jaded once life has slapped them around a bit.
"Is Josh giving you shit because he going through his post-adolescent idealistic phase?"
Dionne on Josh, "Clueless"