Yes, it can constitute age discrimination, because the U.S. Supreme Court ruled in O’Connor v. Consolidated Coin Caterers, Inc. (1996) that an employer can violate the ADEA by firing an older worker in the protected class (over 40) and replaced him or her with a younger worker in the protected class. The court reinstated the ADEA claim of a 56-year-old worker who was fired and replaced by a 40-year-old employee.

In the age-discrimination context, such an inference cannot be drawn from the replacement of one worker with another worker insignificantly younger. Because the ADEA prohibits discrimination on the basis of age and not class membership, the fact that a replacement is substantially younger than the plaintiff is a far more reliable indicator of age discrimination than is the fact that the plaintiff was replaced by someone outside the protected class.