It's the limit of the neocortex.

"The neocortex is located at the outer part of your brain. and it’s in charge of the concept of time like the age of 20 or 30. Cats don’t have a neocortex, unlike humans. So, they don’t get bored or depressed… Even if they have the same food and do the same thing… In their same house in their daily lives. Cats have neither a future or a past. “I’m 20. I’m 30. I’m turning 40 soon.” Only one kind of species on Earth locks themselves up in time. They’re human-beings. Only human-beings use age to make others spend money and draw their emotions. That’s what humans got in exchange for evolution. Whether they’re 30 or 40, it’s the same today for cats." - 이번 생은 처음이라

And that was the moment I realized that I need to seize the day. Might sound crazy to you, but I have gone through my twenty-one years of life doing what people expect, following orders, thinking about others... I have never been brave to do or say what I want. Most of it comes from the oppression of certain things, and even if I'm the type who wouldn't give a damn, I would still consider what other people would think of me and what they would say about me. I want to see a powerful change in myself. I want to see myself know that I am able to do things on my own and not depend on others. It will probably be challenging.