Mary Ann Gant and Banister Hargis married when she was less than 16 years old:

1850 census of Marshall Co MS, family 386:

Banister Hargis, 20, m, farmer, b SC
Adeline Hargis, 16, f, b TN

The family moved to Texas and lived in Caldwell Texas in 1860 (census; family 48):

Banister Hargis, 29, m, b MS
Mary A. Hargis, 26, f, b TN
Geo D. Hargis, 5, m, b TX
Emaline G. Hargis, 2, f, b TX

Adjacent in the census is a listing for John H. Hargis (age 50) and wife Susan (49) and two houses away is a listing for S. M. Hargis (age 31) and wife Vashtie (age 30).

The were still living in Caldwell Co TX in 1870 (census, family 174):

Banister Hargis, 39, m, w, farmer, b SC, cannot read or write
Mary A. Hargis, 35, f, w, keeping house, b TN, cannot read or write
Geo. D. Hargis, 15, attending school, b TX, cannot read or write
Emeline Hargis, 11, f, w, at home, b TX, cannot read or write
Martha Johnson, 35, f, w, at home, b VA

Banister Hargis appears on the Confederate Indigent Families Index (Banister Hargis, Caldwell County).1011