29 CFR § 35.12 - Exceptions to the rules against age discrimination: normal operation or statutory objective of any program or activity.

A recipient is permitted to take an action otherwise prohibited by § 35.10 if the action reasonably takes age into account as a factor necessary to the normal operation or the achievement of any statutory objective of a program or activity. An action reasonably takes age into account as a factor necessary to the normal operation or the achievement of any statutory objective of a program or activity if:

(a) Age is used as a measure or approximation of one or more other characteristics;

(b) The other characteristic(s) must be measured or approximated in order for the normal operation of the program or activity to continue, or to achieve any statutory objective of the program or activity;

(c) The other characteristic(s) can reasonably be measured or approximated by the use of age; and

(d) The other characteristic(s) are impractical to measure directly on an individual basis.