In this world, demi-humans (not just elves or orcs, but also other races like lamias, centaurs, minotaurs, driders, lizardman, alraune, vampires, etc.) have always existed but they did not start living together with humans until certain point in history, which would be beneficial for their coexistence.
The main story would take place in present days (from the 80's until now), in which humans and demi-humans live together in coexistence and are fully integrated into most societies (like workforce, politics, police, sports, etc.), similar to Shepherd0821's Modern Mogal webcomic.
In my case, I had imagined that demi-humans would be seen from time to time during human evolution (which would originate the various mythological legends), but with the increasing territorial expansion of humans, it would conflict with the interests of the demi-humans who wanted to protect their homes and the nature, which would lead them to want to have a role in the humans' political decisions. The integration process would be gradual, more conflicting in some countries, less in others, but would lead to the creations of Natural Rights (humans and demi-humans as having the same rights as citizens) and full coexistence between the humans and demi-humans in current societies.
My question is: in which time of the history would have been ideal for the reveal of demi-humans' existence and the start of their integration into human society?
And the "ideal time" criteria for coexistence is: Moment in history that would have overally less tension, and humanity would be more receptive to demi-humans, I mean, conditions that would lead to future coexistence between all races in present days. Example: JBH quoted in his answer that the invention of firearms would bring difficulties for the integration of demi-humans into society, so this should have started some time before that.
Note: The concept of "Natural Rights" can seen in this post, which also applies here.
First attempt ever at world building, so please, if there are any errors, constructive criticism is very appreciated! Thanks a lot.