Possibly a brother of Isaac Newton Hardin born ca 1836, since they are two years apart in age and both living in households other than their parents in the 1850 Morgan Co. census (Jackson with John Cassity and Elizabeth Brown; Isaac Newton with Isaac Hardin Sr. and Nancy Crose.) One pair of candidates for their parents are Samuel Hardin and Patsy Brown who were married 23 Feb 1825 in Morgan Co. by Samuel Jackson. Theirs is the only other early Hardin marriage in Morgan Co. besides the marriage of Isaac Hardin & Nancy Crose in 1836, and the Brown connection could account for Jackson living with John Cassity & Elizabeth Brown. (There is no Hardin/Cassity marriage recorded in Morgan Co.)

This is not proof, though; it could be that Jackson’s parents were married in a different county.

1850 census of Morgan Co., Ky, lists Jackson Hardin, age 16, living in household #107 with the family of John and Elizabeth Cassity. See their census notes for details.

I have not found Jackson Hardin in the 1860 census yet. Nor have I found a record of his marriage to Julia Donahue, but it probably occured in 1859 or early 1860, judging by the fact that Lafayette was listed as 10 in 1870, and that Julia did not appear with her parents in the 1860 census.

1870 census of Morgan Co., Ky, Blair’s Mill precinct, lists as family #38:
Hardin, Jackson, 36, farmer; Julia, 28; Lafayette, 10; Mary F., 6; Elizabeth, 2; Alfrey, 4/12. All born in Ky.

In 1880 Jackson is listed with his second wife, Lydia Perry, and no children. I have not been able to find Julia or any of the children in 1880, so I don’t know if she died, or if they divorced and she took the children. More likely the latter, since if she had died the children would have probably been listed with either Jackson or some other relatives.