This is a huge part of why I dislike the Polemic explanation for canonicity. You can’t make the entirety of scripture revolve around intertestamental Jewish scholars thumbing their noses at Greek culture without abandoning the larger themes and purposes entirely. The vast majority of events documented in the Old Testament predate even the earliest Greek history.
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If it was a matter of age, why is that principle not repeated and emphasized even more strongly in writings actually contemporary with the prevalence who the specificity of a male child? Pedophilia is cool for a girl, but not a boy? That’s an odd place to draw the line, especially in a time when marriageable age was much younger than now.