I have not been able to find Thomas P. Cornett in the 1850 or 1860 censuses as a child, even though his marriage record (in Grayson Co. in 1862) says that he was born in Grayson Co., around 1841 or 1842 judging by his age. I should look at the Comer records; perhaps his mother had already remarried by 1850, or she might have died young and Thomas been raised by her parents or one of her brothers or sisters.

Grayson Co. marriage register 2, page 9, records the marriage on 25 Sep 1862 of Thomas P. Cornutt, age 22, farmer, single, born Grayson Co., son of Ste. B. Cornutt & Fanny Comer (or Conner), and Nancy A. Clarke, age 20, single, born Grayson Co., dau of A. G. & Delia Clark.

The marriage registers during this period list mothers by their current name at the time of the marriage, so the identification of Thomas's mother as Fanny Comer indicates that she had remarried to a Comer, not that it was her maiden name.

1870 census of Grayson Co., Virginia, Elk Creek township, Independence P.O., page 136, lists as family #52:
Cornett, Thos P., 29, farm laborer; Nancy A., 27; Andrew H., 6; Jno. R., 3; Barbara E., 1; Fannie E., 3/12. All born in Virginia.

Fannie must have died as a child, because she is not in the next census:

1880 census of Rowan Co, Ky, Pine Grove precinct, lists as family #220:
Cornett, Thos P., 38, farmer; Nancy A., 40, wife; John R., 13, son; William C., 12, son; Barbary E., 11, dau. All family members and their parents born in Virginia. (Notice that Nancy’s age is 13 years older than the 1870 census, and Thomas’s is only 9. I don’t know where William C. came from; he was not listed in 1870. Maybe he is a nephew they are raising.)