to take any action otherwise prohibited under subsections (a), (b), (c), or (e) of this section where age is a bona fide occupational qualification reasonably necessary to the
normal operation of the particular
business, or where differentiation is based on reasonable factors other than age; to
observe the terms of a bona fide seniority system or any bona fide employee benefit plan such as a retirement, pension, or insurance plan, which is not a subterfuge to evade the purposes of this Act, except that no such employee benefit plan shall excuse the failure to hire any individual; or to discharge or otherwise discipline an individual for good cause