Arbiter completion (or close to it). imo

"start" of mid game to me is

Dungeon lvl 15

Campaign Nightmare

Daily login 100

and what Trips said

I would personally say its when you can farm dragon effectively (dragon 20) 

But 99% of players will quit before then so not sure if its a good definition.

I think there are many different ways to see this. 

I don't think that Arbiter completion is mid game...for me it is more like early endgame. 

For me it is: Having a full team of 60s....

Running dungeons giving access to 6*-items.

Running Minotaur and Potion dungeons while at the same time leveling other champs.

Getting highest chest of brutal Clanboss.

For me you will reach full mid-game when all of below are true:

Player level: 45

Potion towers: lv 15 100% auto run

Dragon: lv16 100% auto run 

Spider: lv14 100% auto run 

Fire Knight: lvl 12 100% auto run  

Ice Golem: lvl 12 100% auto run 

Faction Wars: auto run on min lvl10 on 50% factions

Clan Boss: brutal max chest with 2-3 keys per day

Missions: Finshed Part 2

D logins: pass 100 day daily reward

I think those will give player good foundation to move forward, (champs arena level, Great hall ommited as those variables depends on individual player)

This topic is debatable. 

But in my own opinion. Finishing Arbiter mission is the end of the tutorial. Thus, putting you at mid game. This is where you improve your teams, work on your faction war teams and slowly climb doom tower. 

The part you call 'tutorial' is a part 99.9% of players who started in the last 6 months will never get

Plariums own stats show 99.5% of people dont even reach level 60, if 99.9% of players dont finish the tutorial then the game has massive issues.

You can not rely on the statistics in their infographic video. There are a lot of players in the video because:

1. Sponsored steam of content creators in twitch. They, themselves have goals to invite several players and required to reach like level 10 only. So they encourage their viewers to create new account, play the game until they reach at least level 10 (I think). Then they can stop playing or create another account in order to reach the number of player goals of the stremer. There is even a counter if you spend in the game right away. I guess it also gives bonus to the content creator.

2. Advertisements in youtube. Raid already become a meme to a lot of content creators in youtube because of their try hard advertisements.

Then of course, not a lot of them will continue playing. They either just want to help their favorite content creator or just looked at the game and did not even bother playing.

So, that explains why it shows very small amount of players reach level 60. The millions you see are not actually who are actively playing. They are probably total no. of accounts made.