A little white lie?

I found a discrepancy in my ancestor, Alvin Theodore Godwin's date of birth. His draft registration card from 1918 says he was born 27 Jan 1878 in Mammoth Springs, Fulton County, Arkansas. His death certificate says he was born 27 Jan 1879 (a year later) in Saddle, Fulton County, Arkansas (which is about 13 miles south of Mammoth Springs). I'm not too worried about the location of his birth as much as the year.

Here is a cropped image of his draft registration card, dated 12 Sept 1918 in which he states his date of birth was 27 Jan 1878 and his age as 40. (Downloaded from Ancestry.com, 3 May 2012)

Here is a cropped image of Alvin Godwin's death certificate, stating his date of birth was 27 Jan 1879 and the informant's name of Alvis Bryson from Mammoth Springs, AR. (Downloaded from Missouri Digital Heritage Site "Missouri Death Certificates.") 

My first instinct is to believe the draft registration card because that is something that my ancestor actually filled out himself, or at least provided verbal information to the draft board about. He would know better than anyone (except his mother and other people who were present at the time he was born) when he was born.

I do not know who the informant of the death certificate was - Alvis Brysom. He could have been a son-in-law, someone who worked in the nursing home, or a friend. My inclination would be towards a family member because according to Alvin's death certificate, he lived in Mammoth Springs, AR which was over 200 miles from Springfield, MO.

However, upon second thought, when I looked at what Alvin's age would have been had he been born in either 1878 or 1879 in 1918, the time the draft board took his information, I found that he would have been 40 years of age if born in 1878 like he told the board or only 39 years of age if he had been born in 1879.

I do not know anything about the laws of the military from WWI, but I wondered if the age of 40 was significant? At least significant enough that he might have lied about his age and made himself one year older? Did he have a less chance of being drafted if he was 40 years of age or older?