Teenagers only know how to play, but don't know how to study hard. When they are old, they regret why they didn't know how to study hard when they were young.

A lazy youth, a lousy age.

"If you don't study hard when you are young, you will regret it when you are old." It is estimated that "if you don't study hard when you are young, you will definitely regret it when you are old." Something like this. It is unscientific to waste one's old age.