Musar su Pirkei Avoth 5:21

And behold [with] people who are righteous and straight of heart, what constantly roars like a lion in their thoughts and growls like the sea is their sins, and their hands falling short from the service of God. For about [the latter] - when a person transgresses, his guilt grows as with weighty sins, like the matter that our Rabbis, may their memory be blessed, said (Yerushalmi Chagigah 1:7), "The Holy One, blessed be He, forwent about [prohibited] sexual relations, but did not forego about the nullification of Torah (wasting time that should be spent in Torah study). Especially since these people's main thoughts and actions are matters of their body and the vanities of their time, they have done much evil. But to the secret of fear, their souls should not come; and their glory should not be associated with the study of their consciences in the images of their hearts and chambers, and their images for fixed times within which they do not give a portion to Torah [study] among their occupations. And the calculation of their souls has been lost from within their hearts, for they are a nation that loses its counsel. And how much are they on the lowest level! And our Rabbis, may their memory be blessed, said (Avot 5:21), "At sixty, old age; at seventy, fullness of years; at eighty, the age of strength; at ninety, a bent body; at one hundred, as good as dead and gone completely out of the world." And their intention, may their memory be blessed, with these words was to warn about repentance - that when a man reaches the days of old age, he should think about his end, if he did not merit to do so in the days of his youth. And since his time is close to coming, he should leave the matters of his body and his desires and rectify his soul. And when he reaches the days of fullness of years, he should add to the removal of [involvement with] the world from his heart. And according to the reduction of years [left], he should reduce his occupation in the world. And he should constantly seclude himself to meditate about fear of God, to think about his soul, to refine his traits and to seek Torah and [its] commandments. And that which they said, "at ninety, a bent (lasuach) body" is from the [same usage as in] (Psalms 102:1), "spill his prayer (sicho)" [and] (Genesis 24:63), "to pray (lasuach) in the field" - as our Rabbis, may their memory be blessed, said (Berakhot 26b), "Isaac established the afternoon prayers, as it is stated, "to pray (lasuach) in the field." For after one reaches ninety years, it is fitting for him that all of his occupation should be with prayer and praises of God, and to speak about His wonders. And King Solomon, peace be upon him, spoke about the matter of the days of old age - that a person should not then be lazy about service to God, may He be blessed. And he said (Ecclesiastes 11:6), "Sow your seed in the morning, and don’t hold back your hand in the evening, since you don’t know which is going to succeed, the one or the other, or if both are equally good." He compared the days of childhood and youth to the morning and the days of old age to evening. And seed is metaphoric for children and for students, as our Rabbis, may their memory be blessed, said (Yevamot 62b), "If he married a woman in his youth, he should marry [another] one in his old age; if he established students in his youth, he should establish [other] students in his old age." For perhaps the children of old age will be more successful in Torah and in the commandments than the children of youth. And likewise the students that he establishes in his old age may be more successful than the first ones. Or they could both be equally good. Afterwards, he stated (Ecclesiastes 11:7), "How sweet is the light, what a delight for the eyes to behold the sun!" Its explanation is that he went back to speaking about the days of old age which he compared to the evening. And since an old man will not taste that which he eats and that which he drinks - like the words of Barzilai the Gileadite - he stated that the old man should enjoy the light of the sun and not be disgusted by it. As disgust obstructs a person from his service to the Heavens. So he should enjoy the light, when he compares it to the days of darkness that are coming - as it mentions in the verse below this (Ecclesiastes 11:8), "Even if a man lives many years, let him enjoy himself in all of them, remembering how many the days of darkness are going to be; the only future is nothingness!" The explanation is that even if a person get very old, let him not be a burden to himself. Rather he should rejoice in all of his years, so that he not lose one of his years and not rest from the service of the Creator. And he should remember the days of darkness, for they will be many. And then he will not be able to perform the service, like the matter that is stated (Psalms 6:6), "in the pit, who can acclaim You?" And the righteous ones overcome their old age, gird their power and renew their strength for the service of God. It is as our Rabbis, may their memory be blessed, said (Shabbat 152), As Torah scholars grow older, wisdom is increased in them." And it is stated (Psalms 92:15), "In old age they still produce fruit; they are full of sap and freshness." And he mentioned above (Psalms 92:13-14), "The righteous bloom like a date-palm, etc. Planted in the house of the Lord, etc." For the righteous are planted in the house of the Lord from their youth, and grow in the house of study from their young adulthood - like a date-palm that blossoms and like a cedar that grows in the Lebanon. And as our Rabbis, may their memory be blessed, likewise said, (Midrash Tehillim 92:14), "'The righteous bloom like a date-palm' - these are the infants, in the way that it was stated (Psalms 144:12), 'For our sons are like saplings, well-tended in their youth.'" And he stated after this about this thing, that they are not [completely] compared to trees. For trees will not give their strength when they age. But the righteous "in old age, they still produce fruit." And King David, peace be upon him, also stated (Psalms 71:18), "And even in hoary old age do not forsake me, God, until I proclaim Your strength to the next generation, Your mighty acts, to all who are to come."