29 CFR 1625.2: Discrimination prohibited by the Act.

It is unlawful for an employer to discriminate against an individual in any aspect of employment because that individual is 40 years old or older, unless one of the statutory exceptions applies. Favoring an older individual over a younger individual because of age is not unlawful discrimination under the ADEA, even if the younger individual is at least 40 years old. However, the ADEA does not require employers to prefer older individuals and does not affect applicable state, municipal, or local laws that prohibit such preferences.