1. People let you down. Sometimes over and over again.
2. You have to clean up after yourself (or pay someone else to). Or things will just stay dirty.
3. Cars need oil. And when your car is low on oil and sounds like a tank but is really just a Toyota Corolla with two children in the backseat, you don’t look cool.
Things grownups may not know yet:
If it is spelled “grownup” or “grown up”. Perhaps when you have grown up you become a grownup?