(8th Cir.2005) (characterizing as an affirmative defense employer’s claim of a bona fide voluntary early retirement incentive plan pursuant to 29 U.S.C. § 623(f)(2)(B)(ii)); Erie County Retirees Assoc. v. County of Erie, Pa., 220 F.3d 193, 199 (3d Cir.2000) (characterizing the RFOA exemption as an “affirmative defense”); Baker v. Delta Air Lines, Inc., 6 F.3d 632, 639 (9th Cir.1993) (stating that “[o]nce the plaintiff establishes a prima facie ease that age was a determining factor in the employment decision, the defendant-employer can rebut the prima facie case and/or assert any number of affirmative defenses [including] the RFOA defense.”); Heiar v. Crawford Co., Wis., 746 F.2d 1190, 1197-99 (7th Cir.1984) (requiring employer to prove that age was a BFOQ); Cova, 574 F.2d at 959-60 (<HOLDING>) (quoting 29 U.S.C. § 623(f)(1)); Laugesen, 510
holding that if the plaintiff makes out a prima facie case of age discrimination the employer must show that the discharge was based on reasonable factors other than age  and if the employer meets that burden the plaintiff must show that age was a determining factor in the discharge holding that the employers discharge of the plaintiff four months after the plaintiff filed a discrimination claim is insufficient to establish a prima facie case of retaliatory discharge holding in an age discrimination case that even though a plaintiff need not revisit his prima facie case after a trial on the merits he still must show that he has met his ultimate burden of proving that the employer terminated him because of age emphasis added holding that adea plaintiff had failed to make a prima facie case of age discrimination when he was replaced by someone only two years younger and within the protected age group and there was no other evidence of age discriminatory motive holding that to make out the fourth element of a prima facie case of sex discrimination plaintiff must prove that males remained in similar positions and similarly for age discrimination that she was otherwise discharged because of her age