Quarterlife Crisis, clarified and simplified:
You are done with school, came out to work with many hopes in life. You think you are going to make a difference in the world... or at least a difference in other people lives... or at the VERY least a difference in your own life. Not quite. Find out the real world is in fact quite routine and boring. Look for a reason to continue this boring and routine life. Start working for only the pay check. Looks at other options. And find none.
Here's my take: that's why people get married from 25 to 30. In crisis, some huge and some small, people grab onto whatever they can grab on. Family just happens to be a good option. The aim from then on is to have a happy family, and this process would take at least 21 years (born child to adulthood).