During the Great War there were many persons who found ways to serve in humanitarian and medical capacities in spite of their ineligibility for military duty. The most numerous of these were the members of the British Voluntary Aid Detachments, familiarly known as ‘V.A.D.’s’, which numbered about 80,000 young women. These volunteers became nursing assistants and hospital workers, and Wikipedia lists twenty-three noteworthy V.A.D.’s, some of whom were Vera Brittain, Agatha Christie, E.M. Delafield, Amelia Earhart, Violet Jessop, and Freya Stark.
Others, including men, were ambulance drivers, stretcher bearers or hospital orderlies. Many were Americans who served before April 1917 or who were under military age. Wikipedia lists sixty such persons who became well-known, including Stafford Cripps, E.E. Cummings, Walt Disney, John Dos Possos, Marjory Stoneman Douglas, Dorothy Canfield Fisher, Ernest Hemingway, Tony Hulman, Ray Kroc, Archibald MacLeish, John Masefield, Somerset Maugham, Maurice Ravel, Robert Service, Gertrude Stein, Pierre Teilhard de Chardin, Alice B. Toklas and William Wellman.
A major source of these medical service volunteers and vehicles was The American Field Service, an organization which still exists and deserves a future entry here.