I just got a notification that one of my riders retired. What does that mean, and what should I do?

Riders will retire at age 65 or when your rider reaches International Level, they may continue to train and show until they reach a certain point, at which they will automatically retire from the game.

You will need to reassign those Rider's horses to other riders.

You can fire that retired rider or keep him hanging around in the riders lounge.