The following information I got from a book titled "John Allen and Phoebe Deuel of Cambridge and Peru NY", written by Charles J. North, 1897. I checked the book out from the New England Historic Genealogical Society's circulating library. They do not show Mary J. Allen as the daughter of John Allen and Phoebe Deuel. If Mary was born in 1820, that would have put Phoebe 63 years old when Mary was born. Of course these self-published books are not always correct, so don't take this as a primary source. Here's what the book lists:
John Allen (1752-1825) s/o Elisha Allen & Elizabeth Fish Phoebe Deuel (1756-1845) d/o Joseph Devol & Rachel Smith Children: David (1773-1839) Prince (1775-1823) Rachel (1776-1872)m. Hiram Prattmy ancestors Elizabeth (1778-1865) Annis (1780-1863) Seneca (1782-1822) Bethuel (1784-1854) Joseph (1786-1858) John (1789-1869) Wilson (1791-1872) Benjamin (1792-1878) Lydia (1794-1881) Martin (1796-1876) Ester (1798-1881) Elisha (1802-1888)
Just as an aside, Phoebe Deuel's line goes back to the Mayflower. Phoebe to Joseph to Benjamin (s/o Joseph Devol and Mary Soule) Mary Soule's father was George Soule Jr, an indentured servant on the Mayflower. The last name of Deuel has changed spellings many times (Deuel/Devol/Davol).
Additional information about John Allen & Phoebe Deuel from the book, it stated that they settled in Cambridge Washington Co. NY on a farm near Elisha, John's father before the Revolution, where three children were born. During the war they went back to Duchess Co. When the war was over they returned to Cambridge, bringing six children with them. They lived there until about 1810 when they moved to Peru, now Ausable, Clinton Co. NY.