Midrash su Salmi 92:15

R. Nehorai says: "I will put aside all the worldly occupation and will teach my son only the Torah; for a man enjoys its benefit in this world and the capital remains for the future world. But other occupations are not so, and furthermore, when a person gets sick when he advances in age, or any misfortune should overcome him, on account of which he could not work, then he is exposed to the pangs of hunger. But the Torah is not so, it protects him from all evil things. Concerning his youth what does the passage say? (Is. 40, 31) Yet they that wait upon the Lord shall acquire new strength; and concerning the aged, what does the passage say? (Ps. 92, 15) They shall still bring forth fruit in old age. So also do we find concerning Abraham, our patriarch (Gen. 24, 1) And Abraham was old — and the Lord blessed Abraham in all things.. We infer that Abraham, our patriarch, observed the entire Torah even before it was given [unto Israel], as it is said (Ib. 26, 5) Because that Abraham harkened to My voice, and kept My charge, My commandments, My statutes, and My laws."

R. Nehorai says: "I will put aside all the worldly occupations," etc. We are taught in a Baraitha that R. Nehorai says: "I will put aside all the worldly occupations and will teach my son only the Torah; for all other occupations are good for a man only during his youth, but when he becomes old, he is exposed to hunger when unable to perform his duty, while the Torah is not so. She stands with the man when he is young and gives him a good and lasting hope in his old age." "Concerning his youth, what does the passage say? (Is. 40, 31) Yet they that wait upon the Lord shall acqiure new strength, they shall mount up with wings as eagles. And concerning the aged, what does the passage say? (Ps. 92, 15) They shall still bring forth fruit in old age; they shall be full of sap and richness."

"They shall still bring forth fruit in old age" (Ps. 92:14). Just as this old age is glory and honour to old men, so shall they be in glory and honour before the Holy One, blessed be He, as it is said, "They shall be full of sap and green" (ibid.). These are the mighty heroes by reason of their good deeds, as it is said, "They shall be full of sap and green, to declare that the Lord is upright" (Ps. 92:14, 15). Why all these (statements)? To declare, and to proclaim clearly the works of the Holy One, blessed be He, for He is righteous and upright, and that there is no unrighteousness (in Him), as it is said, "And there is no unrighteousness in him" (Ps. 92:15).