In the case of WWII such a policy would have put Patton, MacArthur, and Eisenhower (ages 56, 61, and 51 respectively in late 1941) on their sofas in retirement. There are no doubt many examples of generals who were past their prime after fifty, if they ever had a prime at all, whose lack of energy and stodgy ways got men killed and battles lost. But making a certain age a blanket cutoff date for, well almost anything, is absurd.
Not to mention George Marshall.