The Age Discrimination in Employment Act forbids only discrimination against workers over age 40 in the US. It does not prohibit age-based discrimination against a worker who is under 40. Nor does it prohibit discriminating in favor of a worker over 40.

More generally, discrimination in employment is legal by default, unless a specific protected class is created by a specific law. Many classes of discrimination are legal, such as against smokers or obesity.

States and localities may have more restrictions than at the federal level, of course.