Sanhedrin 69: The Very Young Generations of King David's Family
On the age of eligibility to be a ben sorer u-moreh the Gemara stipulates 3 months only, at the age of 13. But the mishnah stipulated physical signs of majority. So the Gemara minimizes the time to say that whichever comes first - the second physical sign of majority OR the end of 3 months. Also, a fairly difficult amud about how young one may have fathered children - in this case, the lineage of David and Solomon, with a lot of math and some extreme conclusions.