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:
I came here to agree with you but then I had the good sense to read the original page which is at the Met (https://www.metmuseum.org/exhibitions/listings/2009/michelan...), and has a lot of background on this painting, including that it WAS actually painted from an existing image (https://en.wikipedia.org/wiki/The_Temptation_of_St_Anthony_(... - worth a look to compare), so my primary skepticism "how could a kid even come up with that" makes a lot more sense that he had an existing image he was copying.
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.