When the Civil Rights Act of 1964 passed, it was a landmark achievement, outlawing discrimination based on race, color, religion, sex, and national origin. But one category was conspicuously absent: age. Congress recognized that age bias was a problem, but its mechanics were less understood than other forms of prejudice. So, they tasked the Secretary of Labor, W. Willard Wirtz, to study the issue.

Wirtz's 1965 report confirmed that age discrimination was widespread, especially against older workers trying to find new jobs. Common stereotypes painted them as less adaptable, less productive, and more expensive. Armed with this data, Congress passed the Age Discrimination in Employment Act (ADEA) in 1967.

The ADEA was designed to protect a specific demographic: workers and job applicants age 40 and older. It applies to private employers with 20 or more employees, as well as to federal, state, and local governments. The law forbids age discrimination in any aspect of employment, including hiring, firing, pay, job assignments, promotions, layoffs, training, and benefits. Essentially, an employment decision cannot be made because of someone's age.

Age discrimination claims typically fall into two categories: disparate treatment and disparate impact.

Disparate treatment is intentional discrimination. It’s when an employer purposefully treats an older worker less favorably because of their age. A manager saying, "we need some new blood in this role," and then hiring a younger, less qualified candidate is a classic example.

Disparate impact, on the other hand, involves a seemingly neutral policy that has a disproportionately negative effect on workers 40 and over. The employer's intent doesn't matter. For instance, a company-wide layoff that targets the highest-paid employees might disproportionately affect older workers, who often have higher salaries due to seniority.

The ADEA isn't absolute. An employer can defend a policy that has a disparate impact if it's based on a "reasonable factor other than age" (RFOA). For example, laying off employees based on documented performance reviews is an acceptable RFOA, even if it affects more older workers.

There's also a narrow exception called a Bona Fide Occupational Qualification (BFOQ). This allows for age-based rules if the employer can prove that age is essential to performing the job safely and effectively. BFOQs are very difficult to prove and are most commonly applied in public safety roles, like for airline pilots or firefighters, where specific age-related physical abilities are critical.

Proving age discrimination became significantly harder after a 2009 Supreme Court case, Gross v. FBL Financial Services, Inc. The court ruled that for an ADEA claim to succeed, the plaintiff must prove that age was the 'but-for' cause of the adverse employment action. This means the employee must show that the action would not have happened but for their age. This is a stricter standard than for claims under Title VII of the Civil Rights Act, where a victim only needs to show that a protected characteristic (like race or sex) was a 'motivating factor' among others. For age claims, age must be the decisive reason.

This higher standard of proof has a real-world impact. An employer can defeat an age discrimination claim by showing they had another legitimate, non-age-related reason for their decision, even if age bias might have played a small part. This makes it challenging for employees to win these cases and underscores the unique legal framework surrounding age in the workplace.

The ADEA fundamentally changed the landscape for older workers, challenging outdated assumptions and providing crucial legal protections where none existed before.