A recipient is permitted to take an action otherwise prohibited by § 110.10 if the action reasonably takes into account age as a factor necessary to the normal operation or the achievement of any statutory objective of a program or activity. An action reasonably takes into account age 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 or characteristics 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 or characteristics can be reasonably measured or approximated by the use of age; and

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