While researching WW1 draft registration cards for Coffey and related families, I have wondered why the ages given by the men at the time of registration were different from their known ages. I have not found a great difference in years, and for the most part differences amounted to a year, sometimes two. But, in rare instances a three year difference has been noted.
Raymond H. Banks has produced a seven-volume work with a very descriptive title: Births, 1873-1900; Part of an Ongoing Compilation of Male Birth Information as Found in the Civilian Draft Registration Cards from World War I. Mr. Banks gave permission to the St. Louis County (MO) Library to reprint the introduction to his work on their website.
He wrote that generally, birth dates given at time of registration are quite accurate, and that any errors were honest. A small number of men did add several years to their age.
Click on the title link to read more, including why an "X" did not always mean the registrant was illiterate. Also included on the Library webpage are links to other research comments by Mr. Banks.