Section 4(f)(2) of the ADEA, 29 U.S.C. ?? 623(f)(2), under a heading captioned ???Lawful Practices,??? set forth an exception to the prohibitions of section 4(a) which allowed an employer ???to observe the terms of 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 . . . .??? (emphasis added).