Birth date information for Mary
1850, Mary Salts is 5 b. 1845
1860, Mary Salts is 12 b. 1848
1870, Mary Salts is 23 b. 1847
1880, Mary Coulter is 30 b. 1850
1900, Mary Coulter is 49 b. 1847

Here are the 1880 and 1900 listings. In 1900, Mary Coulter is listed next to Hettie Salts 36. I think this Hettie may be the wife of James M.'s son Henry A., who you have as dying in 1898.

1880 Census, District 14, Blount County, TN
Family 38
COULTER, Calvin head 44 farmer TN TN TN
COULTER, Mary wife 30 b. 1850 keeping house TN TN TN
COULTER, Hetty J. dau 16 [must be his child by a first wife - she has a baby below] b. 1864
COULTER, Texanna dau 3
COULTER, Sarah C. dau 2
COULTER, John A. son 4months
COULTER, Mary A. granddau 8 months
HEDRICK, Daniel servant 23

1900 Census, District 14, Blount County, TN
Family 79
COULTER, Mary head May 1841 wd 49 TN TN TN 7 children born/6 alive
COULTER, Texanna dau June 1875 s 24 TN TN TN
COULTER, Sallie dau June 1876 s 23 TN TN TN
COULTER, John A. son Jan 1879 s 21 TN TN TN
COULTER, Rebecca B. dau Jan 1881 s 19 TN TN TN
COULTER, Nannie B. dau Mar 1887 s 13 TN TN TN

Family 80
SALTS, Hettie head 36 4 children born/4 alive b. 1864
SALTS, Clemmie M. dau 15 b. 1885
SALTS, Burly son 12 b. 1888
SALTS, Matilda C. dau 4 b. 1896

1910 Census, Mayrville Ward 2, District 9, Blount County, Tennessee
SALTS, Hetty head f w 45 s TN TN TN laundry home
SALTS, Nola dau f w 23 s TN TN TN none [b. 1887]
SALTS, James B. son m w 19 s TN TN TN servent pool room [b. 1891]
SALTS, Matilda dau f w 15 TN TN TN servent bording house [b. 1895]
SALTS, May dau f w 10 s TN TN TN none [b. 1900]