And every man should hasten to repent, and should be afraid that he will be punished for his sin before he [repents], as it is said, ‘Give honor to the Lord your God before it gets dark’ (Jeremiah 13:16), and it says, ‘And remember your Creator in the days of your youth, while the days of evil have not yet come…’ (Ecclesiastes 12:1). And it also is taught in Tractate Shabbath (153A), ‘Rabbi Eliezer says, ‘Repent one day before you die.’ Rabbi Eliezer’s students asked him, ‘Does a person know on which day he will die?’ He said to them, ‘All the more so should a person repent today, lest he die tomorrow. Thence all his days he will be in repentance.’ And our Sages of blessed memory said in the first chapter of Tractace Rosh haShanah: ”Fortunate is the man who fears God.’ Fortunate is the man, and not the woman? It means, ‘Fortunate is the person who repents while he is a man.” And Rashi explains [this means] while he is a young man, in his strength, that is to say, he hurries to recognize his Creator before the days of old age, while he is a mighty man. And even though all repentence is excellent, even in one’s old age, as the verse states: ‘You cause man to repent until contrition (דכא)’ (Psalms 90:3), and our Sages of blessed memory said [this means] ‘until the time when life is crushed (דכדוכא) [i.e. old age]’ (Ruth Rabah to Ruth 3:13), nevertheless, the repentance that a man does during his youth, in his strength, is more desirable and acceptable before God.
(B’rith Avraham, Chapter 5)