This idea sprang out of some discussions I had about the differences between 1e and 2e. Maybe there's some truth in it, maybe it's all just delusion. :) But it's a neat thought...
Looking at the age of Lost Omens, it's clear that magic won't have as easy a ride as it did in previous ages. This is almost Tolkienesque, talking about how things have "diminished" and that's cool. Things change and we have to bridge the gap (pun intended) from 1e to Starfinder somehow.
But what is also kind of cool is to consider what the world was like in ages past, and which rulesets would cover that. If we go all the way back to the days of old Azlant, that feels very much like playing a 3.0 or 3.5 game.
I would suggest PF1 would start with the raising of the Starstone. But where I like to really play with the game is to say that the time in-between Earthfall and the raising of the Starstone.. that's all the old TSR AD&D. Why? Because that game is brutal, especially on low level characters. It's very martial oriented and yes, while casters have some great spells, many of these spells have some serious side-effects (for example, casting Haste ages you a year). Think of that time as a time when the civilized folk lost much of their collected knowledge and had to restart from scratch. As compared to the modern day when there are lots of magical schools, magic is standardized, most of the negative bits are weeded out of spellcasting, etc..
It's a neat thought exercise and there's obviously no right or wrong answer. How would you break it up and with what rulesets?