Our gemara seems very definite that although it is possible that a gentile woman might be able to become a mother at a young age, it is impossible for a Jewish girl. Isn't there a gemara about Bassheva and the young age at which her parents and grandparents produced children? Is that gemara choleik on the braisa of the three women?
The Gemara Sanhedrin 69b discusses the possibility that Bas Sheva and three of her ancestors gave birth when they were eight years old. However, that Sugya is trying to discover whether we can conclude modern (Mishnaic) Halachah from the "Doros ha'Rishonim". The conclusion is that the physical nature of the generations changed. Since then, it is not possible for someone to give birth at that early age, even though Bas Sheva lived only ~800 years beforehand. The Beraisa of the three women did not apply in her time.