It's not taxes, or responsibility, or the mortgage I will someday have (mortgages are the final step in journey to adulthood, you know). It's not even realizing that ten years have passed without you accomplishing anything of note.
It's discovering that, unlike what you were told, it's not the crème de la crème that rises to the top. The intelligent, rational, logical, respectable people are not the ones who are actually in charge.
It's the incompetent people who slide by on a wink and a smile who somehow blunder their way into high places and "run" things.
This is a terrifying, horrifying, demoralizing realization.
There's some part of me that wants to warn my students: I am teaching you to think. You are becoming smarter. You are becoming rational, logical, critical. And this will, almost inevitably, make you miserable: you will not be able to help being frustrated with people who refuse to think. Sheep are blissful in their ignorance; you will not be ignorant, but neither can you achieve that sort of opiating bliss.
The Tree of Knowledge is perilous, indeed.