OMG Cleveland is spamming the forums. O.O
What, no "Survey: When does the Early Game begin?"
For most nations, I could consider the "Mid-Game" to officiallt begin once you have developed 2, separate but synergistic level 4-5 spells that can be used to dramatically increase your military effectiveness.
For some nations, this includes getting Cons4 for thuggery, or even just for Dousing Rods so you can ramp your Blood into high gear, and start getting some serious demon power out.
For a nation of mega-blessed giants, the mid-game might be typified by the moment you say "Oh god I'm no longer completely invincible, why did I take Drain scale??".
A nation with decent research capacity can easily reach this point (with a couple schools at 2-3 for site searching or minor concerns, as well) somewhere between 20-25, and a nation with poorer research pushes it to 25-30, so that's pretty in-line with the aggregate perception of it.