I agree that we have a likely match with your Henry and my John since both living in Knox Co.Perhaps they are brothers or cousins with their b'days only 12 yrs apart.
I also located a group of census records for 1850 Williams Co, OH, Springfield twp which have some name variations of our Willoughbys. The John Willoughby listed below, age 75, may be the son of William Willoughby, ?Birth: 1735 in Monaghan Co, North Ireland, Death: in Kittanning, Westmoreland Co, PA.Christened in Clones, Monaghan Co, Ireland on 11/23/1760 It is believed he left Ireland by way of Newry, Down Co, Ireland and came to America in 1763. A William Willoughby is on the Passenger and Immigration List, Book 8530, Page 95, Longmont, CO Public Lib.
1774 - He settled in Elizabethtown, NJ. Lived in Bethlehem Twp, Hunterdon Co, NJ.
5/21/1778 - enlisted in Cap. John Ross Company, 3rd Regiment, NJ Continental Line from the Hunterdon Co Militia, for a period of nine months
6/30/1778 - joined the regiment
11/3/1778 - deserted,
11/10/1778, rejoined.At the time of his enlistment, he was a resident of Bethlehem Twp, Hunterdon Co, NJ. He was 43 years old, 5'7" in height. Fair complexion and black hair.
1786-1788 - Tax book shows him living in Milford Twp, Cumberland Co, PA. He owned 300 acres & stock.
1790 - Census, Mifflin Co PA, south of Juanita River
11/19/1792 - given 400 acres on Buffalo Creek in Pine Twp (now McCandless Twp),Alleghany Co, PA by act of Congress. The land was in District 8 NW of the Alleghany River, on waters of Buffalo Creek. Confirmed by file at the Dept of Internal Affairs of PA 1798 - Settled on the 400 acres. It is currently the area of Kittanning, Westmoreland Co, PA
Marriage 1 SARAH ---BET. 1759 - 1760 in Clones, Monaghan Co, Ireland.
Henry WILLOUGHBY b: 1765 in possibly NJ
James WILLOUGHBY b: 1767 in possibly NJ
William WILLOUGHBY b: 1768 in possibly NJ
Robert WILLOUGHBY b: ABT. 1770 in MD or PA or NJ
Ann Catharine WILLOUGHBY b: 1 SEP 1772 in possibly NJ
John WILLOUGHBY b: BET. 1774 - 1775 in Elizabeth, NJ
Andrew WILLOUGHBY b: 1776 in possibly NJ
Margaret WILLOUGHBY b: 1785 in possibly NJ.
(I cannot confirm nor deny, at this time, that this Irish WILLOUGHBY family is at all related to our English Knox Co. WILLOUGHBYs.)
?WILLOUGHBY GEORGE 30 M FARMING 800 OH
WILLOUGHBY ABIGAL 30 F PA
WILLOUGHBY JOHN 8 M OH
WILLOUGHBY ELIZEBETH 6 F OH
WILLOUGHBY ANNA 4 F OH
WILLOUGHBY JOHN 75 M 75 NJ
WILLOUGHBY MARY 72 F PA
WILLOUGHBY SARAH 49 F PA
WILABY HENRY 50 M FARMER 1,000 PA
WILABY SUSANNAH 47 F OH
WILABY ANDREW 20 M OH
WILABY JAMES 16 M FARMER OH
WILABY JANE 13 F OH
WILABY ARMSTRONG 12 M OH
WILABY FRANCIS 10 M OH
WILABY CATHERINE 6 F OH
WOOLEBIR HENRY 41 M FARMER 200 PA
WOOLEBIR JULIANN 41 F PA
WOOLEBIR GEORGE 17 M FARMER PA
WOOLEBIR JOHN 14 M OH
WOOLEBIR HENRY 8 M OH
WOOLEBIR SARAH 6 F OH
WOOLEBIR REUBIN 4 M OH
WOOLEBIR JOSEPH 3 M OH
WOOLEBIR HARRIET 3 F OH
WOOLEBIR DANIEL 1 M OH ----TRANSCRIBER'S REMARKS: FIRST NAME COULD BE DAVID
Lastly--a few more research snippets found while looking for some NY ancestors:A David Willoughby and Westel Willoughby are listed in 1810 Newport twp Census, Herkimer Co, NY.Westel and David look like brothers with their similar ages of children, wives and themselves. I understand that Westel's legacy continues with the Ohio town of Willoughby (on Erie Lake)named after him.
Also according to Rootsweb: an Arnold Willoughby, b 9 Oct 1771, died 11 Dec 1832, is another son of Westel and Ruth (Arnold) Willoughby.Arnold and his wife, Catherine Stevens, were living in Norway twp, Herkimer Co, NY in 1810 with their family of 4 children.