You can find several family trees online that indicate my 4th great grandmother, Mary "Polly" (Lyford) Cass was born in Exeter, Rockingham County, New Hampshire either in 1773, 1776, 1778 or even 1783.
I haven't found a birth certificate, nor have I found anyone else that has provided evidence as to where they found Polly's birth year.
The 1840 US Census only lists the head of household, (Abraham Cass) along with a column broken down by age of members of the household. The column "between 60 and 70" doesn't provide enough information to pinpoint the year.
The 1850 US Census states Polly was 72 by the time the census was taken on August 8, 1850, but that appears to be incorrect when compared to the information available on Polly's grave marker. The census is incorrect for the age of Abraham, so it's anyone guess as to whether or not Polly's age was correct during the 1850 US census.
Another problem is verifying the sequence of children born to Polly's parents, Mary (Hardy) and James Gilman Lyford which would help narrow down the possible year of birth.
I have found written documentation* (below) that only provides Polly's name and marriage to Abraham Cass, but it generally lacks dates of both her birth and marriage.
Regarding the marriage of Polly and Abraham, I have seen two consistent dates: 1803 or 1818. It's most likely they were married around 1803 as their first child was born prior to 1805. One of their middle children, Direxia was born in 1814 so it's not likely Polly and Abraham had several children and THEN married.
A little bit of history helps deduce the possible location and date of Polly and Abraham's marriage. My 4th great grandfather, Abraham Cass, and two of his brothers, Theophilus and Simon helped establish Cassville in Stanstead county, Quebec Province, Canada. They were known as "The Nine Partners".
Below is an entry from "Forests and Clearings - The History of Stanstead County, Province of Quebec with Sketches of More than Five Hundred Families" Compiled by B.F. Hubbard 1874
The last paragraph of the history of Cassville provided above may help answer how and when the Cass family migrated to Ashtabula County, Ohio
And from the same book, another confirmation of the marriage of Polly and Abraham, but again, no birth or marriage dates.
The Lyford family moved to Stanstead County, Quebec in 1802. Here is another confirmation (though possibly taken from the same book written by B.F. Hubbard) from "New England families, genealogical and memorial: a record of the achievements of her people in the making of commonwealths and the founding of a nation, Volume 1 by William Richard Cutter - 1911"
Considering Abraham Cass helped to clear Cassville in 1799 with the "Partners of Nine" and then moved their families to Canada in 1800 and Polly's family moved to Stanstead county in 1802, it's probable they married in 1803 and not 1818. And while it's possible Polly and Abraham married in New Hampshire, it's more likely they married in Cassville or somewhere in Stanstead county, Quebec Province, Canada.
*From the "History of the Town of Canterbury, New Hampshire 1727 - 1912 by James Otis Lyford, 1912"