The Sages of the Talmud were convinced that boy could father children at the age of nine years and one day, and on BT Sanhedrin 69 the various legal ramifications of this doctrine are discussed. To the best of my knowledge this presumed age of male maturity is never investigated or substantiated but on Sanhedrin 69b the ante is upped, where we're told that in biblical times children could be fathered by boys as young as 8.
Three attempts are made to "prove" this "fact" using various Tanach passages [See them here]. The first one (ultimately rejected on a technicality) is noteworthy as it rests entirely on 2 Samuel 15:7, and on a word that has caused many brows to furrow. It reads:
וַיְהִ֕י מִקֵּ֖ץ אַרְבָּעִ֣ים שָׁנָ֑ה וַיֹּ֤אמֶר אַבְשָׁלֹום֙ אֶל־הַמֶּ֔לֶךְ אֵ֣לֲכָה נָּ֗א וַאֲשַׁלֵּ֛ם אֶת־נִדְרִ֛י אֲשֶׁר־נָדַ֥רְתִּי לַֽיהוָ֖ה בְּחֶבְרֹֽוAs the Gemarah acknowledges "forty" makes no sense. The plain meaning is that Absalom spoke these words 40 years after his return from self-imposed exile following the murder of Amnon, which is impossible as David only reigned for 40 years. The gemarah's solution is to ignore the plain meaning, and say that it means "forty years since the coronation of Saul" and this dating becomes a significant part of the "proof" that bible heroes fathered children at the age of eight.
And at the end of forty years Absalom said to the king, "Please let me go and pay my vow, which I have vowed to the LORD, in Hebron.
The problem is there's loads of good evidence that "forty" is a scribal error. I checked the LXX and the Syriac: Both have "four." According to Alter, there are two other ancient texts (DSS and Vulgate perhaps? He doesn't say) that also have "four." And "four," unlike "forty," fits the context perfectly.
So, what's a reasonable person to do with this? I count four problems: (1) The Gemarah says boys can father children at age 9. This is false. (2) It says that during an early era, the age of maturity was 8. This is also false (3) No attempt to justify any of this via observation is made. Instead, the Sages looked to passages in a book. (4) One of the relevant passages seems to have been corrupted.
What if the first proof hadn't been tossed out? What if the Sages had paskened l'halacha on the basis of a scribal error? Has that happened? Now what? Or, do we say, "scribal errors be damned, the book that was canonized is the "real bible," irrespective of any errors or additions that may have crept into the text prior to canonization." If the canonizers canonized "forty" then "forty" it is! But if we were to go back in a time machine to the days of Absalom, would we see him speaking these words four years after his return from exile, or forty years after Saul's coronation? Which one is true?
And as for the matter of the 9 year old father, didn't the Sages realize that textual interpretation is subjective, and that a clever reader can make an ambiguous text say almost anything? How could they have believed that the reality of male maturity was based on whatever reading they happened to assign to a verse?
And what about the examples and cases they bring as part of this attempt to prove a boy is sexually mature at age nine The conversation begins with a case of incest - if a mother fondles her child and some preliminary act of penetration occurs - but the Talmud's only concern is whether or not the mother is still allowed to marry a kohen. Nothing is said about the mistreatment of the child. And later, as part of one of the "proofs", the Talmud suggests Batsheva was six when David spotted her from the rooftop and dispatched henchmen to bring her to him for sex. Nothing at all is said about David's lewdness. His crime is passed over in silence. All that seems to concern the Talmud here is the age at which a boy become a man; the incest and the pedophilia aren't remarked upon. Isn't this (forgive me) an example of what more than one critic of the Talmud called (following Matthew 23) straining at a gnat (the child's age) but swallowing a camel (incest and pedophilia)?
(If you haven't caught on yet, the fact that it is altogether impossible to discuss any of these questions calmly and rationally at a public Daf Yomi shiur is the title question's answer)
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