WWI Draft Registrations were completed for approx 98% of the men in the US under the age of 46 in 1917 & 1918. These registration cards have been scanned and searchable images are available on Ancestry (with a subscription). If you have any male relatives living in the US during this time, chances are pretty good that you'll find someone in this database.
Much information can be gleaned from these registrations, which contained the birth date of the individual, who is being supported by them, their occupation, and where they were born. Also, a brief physical description is listed on the second image of the card.
My Grandfather, Leonard C. Anderson was registered in Fairfax County Virginia on June 5th 1917. At the time, he was 21 years old and responsible for supporting his parents (Charles & Sallie (Holt) Anderson), working on their local farm in McLean. He stated that he was born in Fauquier County Virginia, on Feb 26, 1896.
Image: Ancestry.com. World War I Draft Registration Cards, 1917-1918 [database on-line]. Provo, UT, USA: The Generations Network, Inc., 2005. Original data: United States, Selective Service System. World War I Selective Service System Draft Registration Cards, 1917-1918. Washington, D.C.: National Archives and Records Administration. M1509, 4,582 rolls.