Supreme Court and appellate court cases have badly undermined the enforcement of the Age Discrimination in Employment Act (ADEA).
The case with the worst adverse impact on both employees and applicants is probably Gross v. FBL Financial Services, Inc. (2009), in which the John G. Roberts-led Supreme Court held that a plaintiff in an ADEA disparate treatment claim must prove that age was the "but-for" (that is, only) cause of the adverse employment decision, meaning it was the determinative factor, not merely "a motivating factor."
Here’s what it does: