That's actually a the opposite of the truth - big men tend to develop a little later.
Yep, though it depends on what exactly you are talking about. Rebounding and blocks can come fairly early for a big man. Scoring usually doesn't come this fast.
Also, it's a bit of a different game now than the past so I wouldn't project too much based on the past.
Further, Davis and Durant are both very unique players and I think you would be hard pressed to find good comparisons for either from the past.
As good as Davis is, there are some things Durant does offensively that Davis never will. Of course the reverse is true of defense.
22 year old Durant vs a 22 year old Davis, I'm taking Durant bc at that time he had no injury issues where Davis has had quite a few.