"And the grasshopper drags itself along" (Ecclesiastes 12:5) — these are one's feet, which become swollen. "And the caperberry fails" — even desire ceases. All these things come upon the old man. But Abraham, although it is written concerning him, "And Abraham was old" (Genesis 24:1), his desire did not cease, as it is said, "And Abraham again took a wife" (Genesis 25:1).
Abraham Old Yet Undimmed in Desire in Tanchuma
Midrash Tanchuma Buber, Chayei Sara 7:14
ויסתבל החגב, אלו רגליו שהן מתנפחות. ותפר האביונה, אפילו התאוה בטלה, כל הדברים הללו מגיעין לזקן, אבל אברהם אע"פ שכתוב בו ואברהם זקן, לא בטלה תאותו, שנאמר ויוסף אברם ויקח אשה.