Youth is also the best time for serving Hashem, may He be blessed—to be “light as an eagle, swift as a deer, and strong as a lion, to carry out the will of your Father in Heaven” (Avos 5:20). For during the days of youth a man’s strength is rock solid and he can accomplish whatever he sets out to do.
The same is true of repentance. The best time to do teshuvah (repentance) is during one’s youth. Whatever good one does while young, and whatever paths of repentance and self-discipline one develops during that stage of life is the most valued and pleasing to God, inasmuch as it is then that a person’s passions are most powerful and yet he was a mighty man who subdued his evil passions.
Conversely, a youth who sets out on a bad path will develop worse and worse ways, for that is the way of the Evil Inclination: “Today he says to him, ‘Do this’; tomorrow he tells him, ‘Do that,’” etc. (Shabbos 105b). Such is the way of man—whatever one gets used to in one’s youth is what one continues to do in old age. Also, when a person does not take a good path from a young age then even if his eyes are later opened, the task of repentance is so very much more difficult.
Still, even if one did not at first resolve to serve Hashem the way he should have, he should nevertheless gird himself like a mighty man and undertake Torah study, service of Hashem and repentance. He should say to himself: “Let me repent today, for tomorrow I may die.” Death does not distinguish between one who is young and one who is old. Even if someone lives many years, he will never be as young as he is today. So, if not today, then when? “Perhaps tomorrow I will not be able to do what I can do today; and even if I will, won’t I need that day for what needs to be accomplished on that day? But today will come and go and will never return, so how will I ever make up for today?”