1. That by the time most people have saved enough money to travel the world, they are too old to endure such a trip.

2. When your childhood friend who was always better than you at everything is still better than you at everything.

3. That it would be socially unacceptable, at your age, to wrap a Fruit Roll-Up around your pinky and suck on it for two hours.

4. That future generations will look back on the early 2000s as an era in which we did nothing but look back at the 1970s and '80s.

5. Remembering how people of your current age seemed so old when you were a child.

6. When the agony of turning 40 makes you ashamed of how depressed you felt on your 30th birthday.

7. That every time you come to accept your current level of hair loss, there's a more profound stage awaiting you.

8. When your heart skips a beat, reminding you of its eventual failure.

9. Watching the era you grew up in turn from the recent past to retro to history.