There were two posts on GenForum that have interesting information about a John and Mary Culbertson in Pennsylvania. Looks like there might be a brother lurking around, and the location of their grave might be included here. This couple could be John and Margaret's parents, or possibly actually John and Margaret, with some sort of name change (margaret==mary).
Birth and Death Certificates: I can almost assuredly get John's birth certificate from the Vital Records Office or the Philly Public Library, next time I'm in Philly.
One guy posts a lot about Culbertson's in Canada, need to follow up with him.
The 1880 Census exists, but no good index exists. So, I have to page through the entire 1880 Philadelphia census to find a 5 year old John Culbertson. Probably can do the same thing with 1890, as well.