Arpad Elo quotes:

The greatest development after age 21 was shown by Steinitz, who increased his rating by more than a full class interval. Steinitz was the deep student and fierce competitor to the end of his career.

The process of rating players can be compared to the measurement of the position of a cork bobbing up and down on the surface of agitated water with a yard stick tied to a rope and which is swaying in the wind.

Just reading about the various opinions concerning the conduct of the recent championship matches convinces me that the only thing to which two chessplayers could agree is that a third one is over-rated.