9 . The district court did not cite any specific regulations, but the only relevant one is inapplicable here. Although an employer is entitled to distinguish among employees by years of service as long as that distinction is made "without regard to age,” see 29 U.S.C. § 623(f)(2); see also Quinones v. City of Evanston, 58 F.3d 275, 278-79 (7th Cir.1995), that regulation does not defeat the Employees' claim because section 21417 limits benefits on the sole basis of age. 10 . See Hazen, 507 U.S. at 610, 113 S.Ct. 1701 ("[W]e have never decided whether a disparate impact theory of liability is available under the ADEA.... Disparate treatment ... captures the essence of what Congress sought to prohibit in the ADEA.”). See also, e.g., Mullin v. Raytheon Co., 164 F.3d 696, 703 (1st Cir.1999) (<HOLDING>); Ellis v. United Airlines, Inc., 73 F.3d 999,
holding that in title vii disparate impact case an employer may rebut prima facie case of disparate impact by demonstrating that the employment policy at issue is related to the employees job performance and justified by business necessity recognizing cause of action under section 504 based on claims of disparate impact holding that disparate impact claims are not cognizable under the adea holding that there is no disparate impact claim under the adea recognizing oconnors holding as applicable to disparate treatment adea cases