She is listed as age 5 in 1850, age 17 in 1860, age 27 in 1870, and age 37 in 1880. The 1850 date is the odd one out, but it may be the correct one anyway, since the 1850 census also listed a daughter Mary age 7. In 1860 Mary and Louisa are both listed as 17, so they may be twins, but if so it is curious that they were listed two years apart in 1850.

I have not found a marriage record for Ben and Louisa. They probably married in Rowan County, since they were living there in 1870, and marriage records for the 1860’s in Rowan County were lost in a courthouse fire, while marriage records for that period in Morgan and Bath counties still survive.

Her maiden name of Donahue is from the death certificate of their daughter Nellie. Identification with the Louisa who was the daughter of Alfred Donahue and Mariah is circumstantial, based on the facts that the Alfred Donahue family was living next door to the Ben & Louisa Pettit family in 1870, that Louisa Donahue’s youngest sister was named Georgia Ann, and that Ben & Louisa’s daughter Georgia Ann named a son Alfred.

Also a Donohew web site10 in Dec 2001 listed Alfred & Mariah's daughter Lucinda as being married to a Ben Pettit, with no other data (dates, children, etc.) I have found no other evidence of any Ben Pettit in the area with a wife named Lucinda, and the names Lucinda and
Louisa are easily confused. Not very solid evidence, but at least it represents a Donohew tradition that one of Alfred and Mariah's daughters married a Ben Pettit. (It might, instead, however, represent a confusion with Alfred and Mariah's daughter Mary's second marriage, to Ben F. Pettit the son of Andrew David Pettit and Christina Staton, so it might be that neither
“Lu daughter” married a Ben Pettit. The web site did not show any husband or death info for Louisa, and it showed only Mary's first marriage to Owen Crosthwaite.)

1870 census of Rowan Co., KY, Pearce prec., dated 17 June, lists on p. 7/125, family #48 (between the families of Alfred & Susan Donahue at #47 and Isaac & Lucinda Day at #49):
Benjamin Pettit, 40, farmer; Louisa, 37; Leander, 15; Martha, 15; Bluford, 12; John, 2; Thomas Sweeney, 20, farm laborer. All born in Kentucky.

Leander and Martha are bracketed as twins, although they are listed with different ages in 1860. (Some ages appear to be wrong. Note that Louisa is listed as 37 in this census, but as 27 in the 1880 census!

1880 census of Morgan Co., KY, district 87, taken in June, lists on p. 12 as family #106:
Benjamin Pettit, 43; Louisa, 27; Johnnie, 10; Nellie, 8; Charlie, 5; Georgia, 3; Grant, 1. All born in Ky. Both Benjamin & Louisa are checked as being unable to read or write. (Judging by how much the reported ages varied from census to census in this family, they don't seem to have been very good at counting either!)

This family does not appear in the 1900 census of Kentucky, but I do not know whether they had moved out of the state, or both died, or Benjamin died and Louisa remarried. They may have moved to Illinois, where Benjamin and Polly's son Bluford appears in the 1880 census of Putnam Co. (I haven't checked the 1900 census of Putnam Co., Illinois.) On the other hand, three of the children of Benjamin and Louisa married in the 1890's in Morgan or Bath Co., which would tend to argue against Benjamin & Louisa moving away before 1899.