A recipient is permitted to take an action, otherwise prohibited, if the action reasonably takes into account age as a factor necessary to the normal operation or achievement of any statutory objective of a program or activity. An action reasonably takes into account age as a factor if:
(a) Age is used as a measure or approximation of one or more other characteristics; and
(b) The other characteristic must be measured or approximated for the normal operation of the program or activity to continue, or to achieve any statutory objective of the program or activity; and
(c) The other characteristic can be reasonably measured or approximated by the use of age; and
(d) The other characteristic is impractical to measure directly on an individual basis.