OK. I think I have some useful starting answers on how the length of ages is determined from game to game. I ran a bunch of game starts on different settings to see how it affected the range of the Ancient period. I have not experimented with playing into later eras, but my understanding is that they typically have a shorter than the Ancient one. Also, I haven't checked whether age length is affected by the player CIV having a dark, normal, golden or heroic age. Someone who is about to hit an age mark in a save and is close on points might be able to experiment with this. Anyway, here are the results from my settings experiment:
The initial age length range for the Ancient period is only affected by your Time setting. It is the same regardless of whether you are a Settler or a Deity or anything in between. It is the same regardless of whether you or on a small map or a large map. Also the same, in all cases, including various time settings, is that for the Ancient period you must earn 12 event points for a normal age and 24 for a golden age. (Although Aztecs automatically start with 4 event points due to their starting Eagle Warrior.)
Here are the Ancient Age starting range lengths for each time setting:
Online 20-30
Quick 26-40
Standard 40-60
Epic 60-90
Marathon 120-180
I am aware that some players have described games in which the age lengths are fixed, rather than a range. I have not found that to be the case under any setting. I'm not running any Mods except for DLC, so maybe that's an issue?
One other side note from this experiment: I think it's interesting that you have the same time to get the same events score on larger maps that automatically start with more players, since you get event points for meeting players and city states, as well as goody huts. This would seem to add up to higher event score totals and possibly easier golden ages on larger maps, at least those that allow for steady global exploration.