The unprecedented manpower required for World War I fighting forces provoked worries that those men older than 45 might be required to fight, in a way that was previously unthinkable. Yale’s Mentor of Athletics Walter Camp suggested that “Each man should so order his own life as to put himself into proper physical condition.”
“At the bottom of his heart every man who feels the urge of his manhood wants to have an actual part in the actual game. He may be doing his full share in a dozen different ways, his services may be infinitely more valuable along civilian lines than they could be on the tented field, and yet the supreme call may come and he wants to be ready to answer. In the final clash, in the ultimate onslaught of the enemies of civilization, it is the reserves that will count, yes, even to the very last man. And you or I may be that man.”
In other words, get those jumping jacks in.
Making Middle-Aged Men Fit to Help in War: Walter Camp Urges Plan of Moderate Physical Training, on the Plea That Americans Over 45 Years Old May Yet Be Needed
From Sunday, June 24, 1917