Final answer:
The ADEA protects workers aged 40 and over from age discrimination and applies to employers and unions with more than 20 employees.
Step-by-step explanation:
The Age Discrimination in Employment Act (ADEA) applies to public and private employers as well as to unions with more than 20 employees. This legislation plays a critical role in protecting workers who are 40 years of age and older from discrimination on the basis of age in hiring, promotion, discharge, compensation, or terms, conditions, and privileges of employment. Considering the structure of employment in the U.S., where many workers are employed by larger firms, and the diversification of smaller businesses and their employment practices, the ADEA provides a crucial safeguard for an aging workforce, ensuring that employers and unions maintain fair employment practices.