Because of the most recent chapter we know all people must be descendants of the sage of six path as it was said he was the first one to be born with chakra, this proves that its not something that can you can teach someone to harness otherwise more people would of begun to use chakra after his mother ate the fruit.
So now that we know all shinobi descend from his mother using her as a starting point and assuming an average of 2 kids per person and knowing there is 80,000 people in the shinobi alliance if you do log base 2 of 80000 you get 16.28771238 this is the number of generations there must be till you reach 80000 people capable of using chakra so if we round this up o 17 as some people in the era might not be ninjas and you need to anyway to achieve the full 80000.
so we have 17 generations between now and the sage of six paths mother and a generation is defined as 25 years so the sage must have lived around about 425 years ago.
its all an approximation so dont complain about the use of averages.
So now that we know all shinobi descend from his mother using her as a starting point and assuming an average of 2 kids per person and knowing there is 80,000 people in the shinobi alliance if you do log base 2 of 80000 you get 16.28771238 this is the number of generations there must be till you reach 80000 people capable of using chakra so if we round this up o 17 as some people in the era might not be ninjas and you need to anyway to achieve the full 80000.
so we have 17 generations between now and the sage of six paths mother and a generation is defined as 25 years so the sage must have lived around about 425 years ago.
its all an approximation so dont complain about the use of averages.