This site includes a photo of a grave marker that indicates she was born in 1805. https://www.findagrave.com/memorial/143645116/sarah-margaret-eldridge However, the marker looks recent, so perhaps it was based on inaccurate information.
Other sites list different birth dates (1801, 1806...) http://yeahpot.com/boggs/elijah1784.php http://danielscheer.com/family/familygroup.php?familyID=F9429&t...
I'm guessing that the 1801 year comes from the 1850 census, where she's listed as 49 years old. https://www.familysearch.org/ark:/61903/1:1:M8DH-P66?from=lynx1UIV8...