When I have used the SSDI to check death dates, I have found them to match other sources I had, such as family memory and/or death certificates.  It may not be a reliable source for age for the reasons that Peter Cohen mentioned. I have at least three ages for my paternal grandfather, for example, not including the SSDI, which did not exist when he died in 1917.  I had to go with the preponderance of the evidence, giving some weight to the death certificate, presuming the information was given my grandmother.  
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Barbara Sloan
Conway, SC