In one of my games, I had a level 3 Magnar attempt AND complete the Arena of the Slakhanan on turn 20. I do not want to spoil too much, but this is a extremely hard quest (at least Deadly, maybe Epic) with an awesome reward. Hell, the second best reward is awesome. Magnar should not have had a snowball chance in Hell to complete it.
He got 3 levels and I would not be surprised if he got the reward as well. This is a major issue. I have seen the AI wander around with Ashwake dragons and high level heroes from quests as early as turn 50, and now I know why.
There are many reasons why this is a bad thing: quests and their rewards get removed from the map too early, the AI is advancing too quickly, and finally, the AI can very early get something with which no player can deal.
Please, this cannot be too hard to fix!