Here in NE Connecticut family cemeteries are fairly common and there's one located on the property out behind our 1790's farmhouse
I've started a "tree" of the family buried there and while we've lived here awhile, I've never noticed until the other day that the original mother(Selinda) of the family supposedly died before two of her three children were even born.
There was a second wife (Betsy) with additional children but that's not the same issue.
The most likely solution is that there's mistakes with the dates on the headstones. But Selinda's 1820 gravestone is "period". Slate with a urn and willow design unlike any other in the graveyard and so is likely to be much older than the youngest daughter's stone (Celinda, daughter of Celinda) who died at age 16, TWENTY FOUR years later
There also could have been a third (middle) wife with a similar first name, (Celinda) but there's no stone for such a person and it seems highly unlikely.