Federal law protects state and local government workers from discrimination based on age regardless of the size of their employer, the U.S. Supreme Court held Nov. 6.

The 8-0 decision ensures that tens of thousands of first responders and other employees of state political subdivisions can’t be denied hire, fired, or otherwise mistreated because they are 40 or older. The federal Age Discrimination in Employment Act covers all public-sector workers, even though it only applies to private-sector employers with 20 or more employees, the high court said.

The justices sided with the U.S. Court of Appeals for the Ninth Circuit’s...