Samuel and his wife are buried in the town of Chautauqua. He dies 18 Oct 1863 and his head stone lists his date of birth as 20 June 1787. His wife, Mary Justina, dies on 14 Nov 1848 (she is listed as his wife on the head stone). Her age at death is recorded as 68 years 7 months 14 days. Using my handy date of birth calculator, I compute her date of birth as 31 Mar 1780. I wonder if the census taker placed her in the wrong column on this record?
The reason I know the female born 1815-1820 is not my Nancy Porter is because of the 1850 census along with a small passage in the History of Chautauqua County, New York. The 1850 census from the town of Chautauqua, is shown below.
Samuel Porter, aged 63 is living the household of Robert Huse (actually spelled Hewes). There is also a 15 year old female named Hortensey Porter living with them, too. The section from the The History of Chautauqua County, New York is displayed below.
So we know that Samuel is living in the house of his son-in-law Robert Hewes. Olive is 30 years old in 1850 making her born circa 1820. I think it's a safe bet to say the female born 1815 to 1820 in Samuel Porter's household is Olive.