When one is young his physical strength is dominant, but as he gets older, his physical strength leaves him, as do his desires. At this time he can rejoice in his intellectual achievements. One who invests in the intellect and rejoices in it will be able to yearn for God and desire to connect to Him in old age.

The philosophers clarified that during the period of youth, physiological forces are an obstacle to reaching almost all levels of virtue, and all the more so to acquiring the clear thinking that comes from perfection of the mind and that causes one to desire God, may He be exalted, as this is not possible when the bodily fluids are boiling. However, as physical strength weakens and the fire of one’s desires fades, the intellect grows stronger and its light spreads, and its understanding is purified, and one rejoices in that which he has understood [and the desire for God increases]. (Rambam, Guide of the Perplexed III:51)

Further reading: For more on the Jewish attitude toward the elderly, see A Concise Guide to Halakha, p. 612.

If one chooses a good path in his youth and does not submit to physical desires, then as the years pass and his physical strength leaves him, his mind becomes serene and he rejoices over the fact that he chose the good. In contrast, if one chooses to follow worldly pleasures in his youth, and subdues the voice of the intellect, then as the years pass and his physical strength leaves him, his mind is unsettled and he cannot make peace with the choices of his youth.

The minds of older Torah scholars are settled:

“Ignorant people’s minds grow unsettled as they age, but the minds of Torah elders become composed as they age” (Mishna Kinnim 3:6). This does not mean that as they become older their wisdom increases, as this may not be the case. Since the mind requires physical vessels, it is possible that when someone becomes very old, those vessels will become very weak, and the mind too will become weak… Rather it says: “Their minds become composed,” and the explanation is that Torah elders, who in their youth set aside worldly appetites and chose the good path, become more and more content as they grow older, and their minds are at ease with regard to their choices.

Young people experience conflict between their reason and their physical desires:

This is because it is impossible that their immaturity does not cause them occasionally to submit to temptation in their youth, even though they have committed to renounce worldly appetites and excesses and they know they are not behaving rationally. Torn as they are between these opposing forces [reason and appetite] their minds cannot be entirely settled. However, once they begin aging and their appetites weaken, and they perceive that whatever they imagined concerning matters of this world is totally null and void, fleeting and insignificant, then reason is not in conflict [with that perception], and they are settled of mind because they see clearly that they chose the good and were committed to it, and all the pleasure they took in the world is meaningless, so they should not regret having given it up.

Ignorant people are unsettled in their old age.

However, the opposite occurs to ignorant people; in their youth they are tempted by their worldly desires, and their minds are susceptible at that time, and at ease, and they are not concerned about the fact [that their behavior] is irrational. As they age, and [pleasure in] the [physical] world slips away from them, and they from it, they perceive that all of their appetites and preferences in the days of their youth were nothingness and emptiness… because of this their minds are unsettled with regard to their former lifestyle. (Rabbeinu Nisim Gerondi, Derashot HaRan 10)