This UpLaw overview maps how federal age discrimination law operates in the American workplace, anchored in the Age Discrimination in Employment Act (ADEA) and its 1990 amendment, the Older Workers Benefit Protection Act (OWBPA). It identifies who the statute protects (workers 40 and older), which employers must comply (generally those with 20 or more employees, and every political subdivision), and the two proof theories that carry nearly every case: intentional disparate treatment and facially neutral disparate impact. It works through the demanding but-for causation rule the Supreme Court imposed in Gross v. FBL Financial, the reasonable-factor-other-than-age defense from Smith v. City of Jackson and Meacham, the rigid severance-waiver and reduction-in-force disclosure rules the OWBPA layers onto separations, and the harassment and retaliation claims that travel alongside the core ban. It also details the unusual opt-in collective-action mechanism, the EEOC charge process and its short deadlines, the economic-only remedies the ADEA allows, and the many ways state law reaches further than the federal floor. Invented hypotheticals and a plain-language FAQ make the doctrine usable for workers, managers, and counsel.