When indexes are created, indexers are instructed to record information exactly as it appears on the original record. Humans do occasionally err, but it is important to remember that the error could have been done by the informant or the clerk filling out the record.
A search of the World War I Draft Cards at Ancestry indicates that 10,893 individuals in the database were born in 1918. There has to be something amiss someplace. Registrants for this draft could not actually have been born in 1918. My unscientific study of some of these hits failed to locate one card that did not say the registrant was born in 1918.
How could this happen? There are many reasons, but obviously none of the men were actually claiming to be under the age of 1. Registering men for a draft when war might have appeared imminent could have lead to some distraction on the part of the registrars. After all, how many of us today have accidentally put the incorrect year on a check when writing one?
It is important to keep this in perspective. There were approximately 24 million registrants for this draft–10,893 only represents .045% of the total—a small percentage to be certain, but enough to consider if you can’t locate your ancestor when searching by year of birth.
When searching any database, consider that one of the pieces your ancestor gave could have been either given or recorded incorrectly. It will impact how he appears in the database. Try altering or omitting one search term at a time. You may be pleasantly surprised at the results even if his age is correct.
-Taken from Ancestry.com