The federal Age Discrimination in Employment Act is structured differently from Title VII. That is why case law interpreting Title VII is not always applicable in ADEA cases.
Like Title VII, ADEA authorizes "disparate impact" cases - where the plaintiff demonstrates a neutral employer practice that has a greater effect on members of a protected group than non-members. The statute provides that an employment practice is legal if based on "reasonable factors other than age." The question the Court addressed in Meacham v. Knolls Atomic Power Lab. is whether the "RFOA" provision is part of the plaintiff's burden of proof or an affirmative defense. Agreeing with the EEOC's long-held position, the Court held that employers have the burden of proving that a challenged practice, one that has a statistically significant impact on older workers, is based on "reasonable factors other than age."