More accurately where you were born and when your SSN was issued (my brothers who are four years older than me got their SSNs at the same time I did). Some of us older folks were born in an era in which you didn't automatically get an SSN along with the birth certificate. And then there are people who weren't born in the US so will have had their SSN not matching birth year.

I also think it's less than 5 of the 9 digits that are reflected in this manner. That would not leave room for a lot of distinction in SSNs.