Notwithstanding any other provision of this subchapter, it is not unlawful discrimination on the basis of age or disability for any employer, employment agency or labor organization to observe the terms of a bona fide seniority system or any bona fide employee benefit plan, such as a retirement, pension, or life or health insurance plan, any of which is not a subterfuge to evade the purposes of this subchapter. No employee benefit plan, however, excuses the failure to hire any individual. No seniority system or employee benefit plan shall require or permit the involuntary retirement of any individual because of age. Mandatory retirement on account of age, necessitated under a police or firefighter retirement system, is specifically authorized. (Added 1981, No. 65, § 4; amended 1999, No. 103 (Adj. Sess.), § 3.)