The verse (Tehillim 127:4) states: “Like arrows in the hand of a mighty man, so are the sons of one’s youth.”
On this, Reb Mendel of Kotzk stated:[1]
“Just as after one shoots an arrow, no amount of regret will help to get it back. So too, the regret of the old will do nothing to return one’s youth.”
[1] Emet Mkotzk Titzmach, Pg. 18