Age discrimination involves treating someone (an applicant or employee) less favorably because of his or her age.
The Age Discrimination in Employment Act (ADEA), according to the US EEOC only forbids age discrimination against people who are age 40 or older. It does not protect workers under the age of 40, although some states do have laws that protect younger workers from age discrimination.
It is not illegal for an employer or other covered entity to favor an older worker over a younger one, even if both workers are age 40 or older.
Discrimination can occur when the victim and the person who inflicted the discrimination are both over 40.