Deuteronomy 29:18 – And it will be, when he [such a person] hears the words of this oath, that he will bless himself in his heart, saying, “I will have peace, even if I follow my heart’s desires, so that the watering provides also for what should remain thirsty”
Gemora Sanhedrin 76b: Rav Yehuda said in the name of Rav: One who marries his daughter to an old man or one who marries his young son to an older woman and one who returns a lost object to a Kuti, about him is the verse written “in order to add the unintentional sins to his intentional sins, God will not willingly forgive him.”
Torah Temimah – Colloquial Translation of Note #17:
The word “ספות” in the verse has the meaning of join [“also” in the above translation] as in the verse in Isaiah 29:1: Add year to year. An older person is referred to by the word “watering” because he is already satiated with life and the young girl is thirsty for life. The same applies for an older woman marrying a boy. Following this path can lead to unintentional transgressions.
Observe what Rabbi Yaacov Emden has written on this topic in his notes on this section of gemora. He takes a new point of view. He writes that the issue of old age and youth discussed in the gemora is not an issue of the number of years that a person has. Rather it is more an issue of the health of a person. In general, he notes, an older person has gotten weaker and a younger person is healthy. Therefore, Reb Yaacov states that an older person who is healthy would not have any prohibition in marrying a younger person.
But according to what I [the Torah Temimah] wrote above the reason for this prohibition is due to the contrast in feeling the fullness of life of the older person versus the thirst for life of the younger person. This is according to the words of the verse “satiation / thirst”. One should not take these words [of the gemora] out of their simple meaning. “Old man” means “old man” and “young girl” means “young girl”.
Regarding the topic of returning a lost item to a Kuti, please see my comment on Deuteronomy 33:2
Translator Note: I do like this note of the Torah Temimah as he challenges Rabbi Yaacov Emden’s opinion on the appropriateness of marrying an older man to a young girl. As far as the issue of returning a lost object to a Kuti, I hope to translate that note on verse 33:2 at some future date. The translation of the verse, by the way, is a combination of translations, the second half of the verse is translated according to Samson Raphael Hirsch English Chumah (copyright 1966).