So, if you're like me, popped everything else, and somehow missed this, here is how I got it.
Set up a skirmish and set it to Supremacy (you are unable to get past age IV in a "default" deathmatch). Set the starting age to Nomad (others have reported that it doesn't pop unless you start here, thanks @ObeyedDragon!).
Once you're in, click on your Town Center and keep advancing your age. To advance to the Imperial age, I believe you will need 4,000 Food and 4,000 Gold.
To make things go quite a bit quicker, press Enter and submit the following cheats:
A whole lot of love
-- This will give you a ton of resources
Speed always wins
-- Instant build/research
Once, I advanced to the Imperial age, it popped for me immediately. Though, I have noticed, I will get the notification on my phone (through the Xbox app) significantly faster than I will on my PC (if at all).
If you see a way to improve this guide or want further clarification, leave a comment! Thanks!