When the world should be your oyster, but is really your Inferno. A period of time, right after high school to late twenties, where you are expected to perform and act exactly like the other adults who are ten years your senior or older, yet more than likely no one has taught you the basic ideas and attitudes of how to be successful in the real world. You will also find out that many of the well intentioned adults, or the adults who wanted to feel important, (teachers, parents, etc.) who actually gave you advice or put in there two cents when it wasn't asked for, were down right dumb, if not delusional, and had no clue what the hell they were talking about when it comes to being successful in real life.
So you are left to figure out much of it on your own. Most of the areas of your life you will be confused about are dating, career, finances, finding a place to live, and friends. This is more than likely going to be the worst time in your life.
*THIS IS NOT A JOKE* This will make the on-going, never ending, day to day, living hell that was high school or middle school seem as easy as breathing. You will more than likely wish you were still in high school, middle school, or 10 years old back in elementary school...just so that you'd be sheltered from and never have to know the never-ending mental turmoil you are going through.
If the quarter life crisis has taught me one thing above anything else, it's that when I have kids I am always going to be open to listen, without judgment, (unlike the assholes I grew up around) when they need me to and that teaching them about bills, check books, credit cards, bosses, job applications, relationships and all that other happy horse shit isn't such a bad idea either.