The accuracy of the original should never be assumed--regardless of when the record was created and who gave the information. And sometimes errors are just that--errors.
If you search the online index of World War I Draft cards at Ancestry.com
for only the year 1918, you will get many hits. If the actual card image is viewed, it will be seen that the card does actually say 1918 as the year of birth. Think about it for a second, have you ever written the wrong year on a check or an application? It is very easy to do. Those who transcribed the data for the online index of World War I Draft cards at Ancestry.com
were supposed to transcribe what the card said, not what they thought it should say. Of course, the transcribers were human so they may have made mistakes, but the mistake could just as easily have been on the original.