Coaches are similar to the players - some go up and some go down. Most younger coaches tend to go up over the seasons and throughout the season (older coaches tend to go up, down, or stay same). I keep track during the season as to which coaches go up or down. If you click view (on coach card), click on progress, you can see which way he is trending.
As for retiring, when they hit around 53-55 years of age, that's when I start to worry. However, some will continue for many more years. You never know when they will call it quits.