What was the youngest age at which a prodigy could be considered to have been a master? There is a photo of Reshevsky playing at a very young age (like 8) against many adult players but there were no ratings in those days. I assume that a modern GM who got the title at 12 must have been close to master strength at 8 but I am not sure this is true. I wonder in fact if there is some limitation of the human brain that prevents a certain level of play before a certain age in the same way that no child can run a 4 minute mile, etc.
Let's just say a master is someone who crosses the 2200 USCF mark to get a rough idea. In that case, Abhimanyu Mishra and Liran Zhou both crossed the 2200 barrier around 9 years and 3 months. Christopher Yoo did the same thing in just under 10 years. Note that these chess players did not necessarily reach 2200 strength at those ages, it is likely they were weaker than 2200 but from stochasticity, achieved the crossing. Also note that USCF is the federation associated with United States and 2200 USCF corresponds to something like 2100 FIDE.