> It seems massively unlikely.
Why? There were other talented people who produced masterful works at an early age. From the same time as this there's a Dürer self-portrait, also aged 12-13:
https://en.wikipedia.org/wiki/Self-Portrait_at_the_Age_of_13
> We don't have any really reliable records from that time.
Uh, no. There's no documented attribution of that painting to Michelangelo; that doesn't mean that other things weren't reliably recorded.