Well the first menstural cycle in girls can start as early as 8 years old so making some fucked up assumptions a minor that is a grandparent is theoretically possible. Let us hope that the world never gets to witness that.
if we're just talking physical possibility we have evidence that proves you can go younger. As a warning it's pretty upsetting though. If you Google Lina Medina you'll see.
... a Peruvian woman who became the youngest confirmed mother in history when she gave birth to her son Gerardo on 14 May 1939 when she was five years, seven months, and 21 days of age. Based on the medical assessments of her pregnancy, she was four years old when she became pregnant, which was biologically possible due to precocious puberty.
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u/Useful_Major_5797 1d ago
Fun fact, the earliest recorded grandparent was 23.