A more thorough explanation:

The Age Discrimination in Employment Act (ADEA) is a law that protects workers over the age of 40 from being discriminated against in the workplace. This means that employers cannot treat older workers unfairly because of their age. However, the law does not protect younger workers from discrimination.

If an older worker believes they have been discriminated against because of their age, they can try to prove it by showing direct evidence of discriminatory intent or by meeting the "McDonnell Douglas" test using circumstantial evidence.

Direct evidence is when there is clear proof that the employer discriminated against the worker because of their age. For example, if an employer wrote on an application form that the applicant is "too old" for the job, that would be direct evidence of age discrimination.

If direct evidence is not available, the worker can use the "McDonnell Douglas" test. This test requires the worker to show that they are over 40 years old, that they were doing their job well, that they were fired despite doing their job well, and that they were replaced by someone younger.

If the worker can prove a prima facie case of age discrimination, the employer must then provide a legitimate, nondiscriminatory reason for the employment decision. This could be something like misconduct on the job or a lack of required qualifications.

After the employer provides their reason, the worker can try to show that the reason is not true and that the real reason was age discrimination. This can be difficult to prove, but if the worker can show that the employer's reason is not true, they may be able to win their case.

An older worker named John believes he was fired from his job because of his age. He thinks his boss wanted to replace him with a younger worker. John does not have any direct evidence of age discrimination, so he decides to use the "McDonnell Douglas" test.

John is over 40 years old, and he was doing his job well. However, he was fired and replaced by a younger worker. John believes this is evidence of age discrimination.

John's employer says that they fired him because he was not doing his job well. They say that they had to replace him with someone who could do the job better.

John now has to try to show that his employer's reason is not true and that the real reason was age discrimination. He might do this by showing that his performance reviews were good or by showing that the younger worker who replaced him was not actually better at the job.