Yes probably first known work. The salient point though is that he did this at 12.

"As a young boy, Michelangelo was sent to the city of Florence to study grammar under the Humanist Francesco da Urbino.[13][16][d] Michelangelo showed no interest in his schooling, preferring to copy paintings from churches and seek the company of other painters."

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https://en.wikipedia.org/wiki/Michelangelo#Early_life_and_ca...

It seems like this painting would be from his time at Urbino's grammar school. His first apprenticeship started when he was 13. You might expect a renaissance artist to do a lot of work on their master's paintings (detail work, etc.) before ever putting their own name to a canvas, but this is, apparently, Michelangelo doing his own thing before ever being apprenticed.

So, while he likely did paint things before this that didn't survive, it's pretty amazing that this is the work of a kid who has yet to be apprenticed and is just pursuing it on his own with nothing more than the advice of people he was hanging out with.

and also keep in mind, you probably make many sketches before putting brush to canvas...