This sentence comes from the Analects of Confucius: Confucius said: "I am determined to learn from five out of ten, stand at thirty, be puzzled at forty, know my destiny at fifty, listen to my ears at sixty, and do whatever I want at seventy."
Translation:
Confucius said, 15 years old is determined to study, 30 years old is independent, and 40 years old is not confused by foreign things. At the age of 50, I understood my destiny. At the age of 60, I don't feel uncomfortable treating all kinds of remarks correctly. At the age of 70, I do whatever I want and don't overstep my bounds.
15 to 20 years old is called "Elephant Dance Year"
So at the age of 20, I know my destiny, and I feel affected when I say this. There is a feeling of painless groaning.