Individual Details

FamilySearch family tree suggests a first marriage, 2 Nov 1845, in Clermont, Ohio, for Nathaniel to Nancy West, b. 6 Feb 1826, Clermont Co, Ohio, dau of Alfred West and Elizabeth Archer. I suspect Nancy died before 1870. There is no such marriage listed in the Ohio marriage database on FamilySearch.

1850 Census. Posey, Clay, IN, Hh 17
Nathaniel Allen, 27, Farmer, b. IN. Nancy, 24, b. Ohio
Miran, 2, Elizabeth 6 months.

1860 Census. Posey, Clay Co IN, Hh 682. Living next to his parents.
Nathaniel Allen, age 37, Farmer, b. IN. Nancy, 34, b. Ohio
Myron H., 12, Elizabeth 10, Mary 8, Clara 4.
Benjamin F. West, age 22, Farm labor. [Probably Nancy's younger brother]

In 1870, Nathaniel & Nancy's children were all living together in Posey, Clay Co IN, without their parents, Hh 159
Myron Allen, age 22, Working farm. Elizabeth, age 21. Mary, age 16, Clara, age 12

I found this marriage in Vigo Co, although I never found an earlier marriage for Nathaniel, nor did I find his marriage to Ada E. Kelly as in the 1880 census.
Nathan Allen to Nancy Russell, 21 Dec 1873, J. T. Russell, MG

Family Search shows that Nathaniel married Eva Ada (Rockwell) Kelly, 24 Nov 1870 in Salt Creek, Ohio. (there is a Salt Creek Twp in three different Ohio counties...)
The marriages in Ohio are a bit strange and to date I have found no confirmation - he always lived in Indiana.

1880 Census. Terre Haute, Vigo Co, IN Hh 126
Nathaniel Allen, age 57, Agent, b. IN, parents b. NY
Ada E., wife, age 53, b. Ohio, father b. Maryland, mother b. Vermont
Carrie, age 23, daughter, b. IN, father b. IN, mother b. Ohio [Is this Clara?]
Wm P. Kelly, age 17, stepson, clerk, b. IN, parents b. Ohio
Chas. A. Kelly, age 16, stepson, operator, b. IN, parents b. Ohio

1900 Census. Harrison Twp, Vigo Co, IN Hh 215
Charles A. Kelly, b. Dec 1862, age 37, b. IN, parents b. Ohio, insurance agent
Lulu, wife, b. Mar 1865, age 34, b. IL, parents b. Germany
Charles A. Jr., son, b. Sept 1891, age 8, b. IN
Flickinger, domestic, b. Sep 1877, age 22, b. IN, parents b. Ohio
Nathaniel Allen, G.Father, b. Feb 1822, age 78, b. IN, parents b. NY
E. Ada Allen, mother, b. May 1825, age 63 [wrong-she was 75], 2 children, both living, b. Ohio
Amelie Rhele, mother in law, b. Apr 1835, age 65, 5 children, all living, b. Germany, parents b. Germany

The Evening Gazette, Feb 2, 1904
The Oldest Person Born in This City
Nat Allen, Who Was Born in Terre Haute in 1822 Will be 82 Tomorrow.
Is Still Hale and Hearty
His Birthplace was on First Street Between Ohio and Walnut - the House Long Since Disappeared.
Nathaniel Allen, a well-known resident of Terre Haute, will be 82 years old tomorrow. He was born in Terre Haute on Feb 3, 1822, and is the oldest person living who was born in Terre Haute. The house in which he first saw light of day was a small three room frame house that stood on the East side of First street between Ohio and Walnut and which has not long since been torn down. Mr. Allen has seen Terre Haute grow from a village to a large and flourishing city.
Mr. Allen's parents were Ira Allen and Lydia Cargill. The former died in 1869 in Clay county and the latter in Terre Haute in 1871. Mr. Allen has three children, as follows: Myron H. Allen, Bozeman, Montana; Mrs. Mary Hussey, San Jose, Cal., and Mrs. Clara Wilson, Butte Montana. Mr. Allen has twelve grandchildren and nine great grandchildren living. The oldest of the latter is about 15 years, the youngest, four months.
Mr. Allen was married twice. His first wife, Nancy West, died in 1869. His second wife, Ada E. Kelley, whom he married in 1870, is still living.
Mr. Allen, who lives at 222 South Thirteenth and a half street, is still hate and heavy. He is connected with the insurance firm of Kelley and Wagner.
Mr. Allen was born when James Monroe was president, seven years after the battle of New Orleans and seven years after the battle of Waterloo. Florida had only been purchased three years from Spain. He was two years old when Lafayette visited the United States. Only six years before his birth Indiana was admitted to the Union. Napoleon had only been dead a year when he was born. He was 24 years old when war broke out with Mexico.

1910 Terre Haute City Directory
Nathaniel Allen, lived at 1101 S. 7th

Indiana Death Certificate #19
Nathanial Allen died 6 Feb 1912 of cerebral hemorrhage. His residence was 1121 S. 7th St, Terre Haute, Harrison Twp, Vigo Co, IN. He was a widower - his wife having been Addie Allen. Born 3 Feb 1822 in Terre Haute. His father was Ira Allen, birthplace unknown; his mother was Lydia "Cahill", birthplace unknown. C. A. Kelly was the infomant. Nathaniel was buried Wood Lawn Cemetery, Terre Haute, on Feb 8th.

Found in the Indiana WPA Death Records. Nathanial W. Allen, age 90, d. 6 Feb 1912, Terre Haute. This is the correct age, so presumably the correct Nathaniel.
He is buried Woodlawn Cemetery, Terre Haute.

Birth	3 Feb 1822	Indiana
Marriage	24 Nov 1870	Ohio - Eva Ada "Addie" Rockwell
Death	6 Feb 1912	Terre Haute, Vigo County, Indiana
Marriage		Nancy West

Spouse	Nancy West (1826 - 1869)
Child	Myron Allen (1847 - )
Child	Elizabeth Allen (1849 - 1904)
Child	Mary Allen (1852 - )
Child	Clara Allen (1856 - 1934)

Spouse

Eva Ada "Addie" Rockwell (1827 - 1904)

Father	Ira Allen (1799 - 1866)
Mother	Lydia Cargill (1797 - 1876)
Sibling	Edward B. Allen (1832 - 1901)
Sibling	Louisa Caroline Allen (1835 - 1910)
Sibling	Peter B. Allen (1841 - 1896)