If he was 21 in 1870 and 84 in 1930 the gap is 5 years + or -. If he served in the Civil War he was probably under age and was discharged in 1862 or later because of the enlistment laws. My bet is you may never know his exact age. He was some where between 79 and 84 maybe. If he served for a period whether discharged, some counties/parrish Judge may have granted a pension to him or his widow. The two census are your anchors. The pension will help, but the Texas units are out and I will bet the 3rd and 6th Louisiana Cavalry are out also. I looked for links in battles and the Louisiana units did not link with the Texas except for one or two battles in the Atlanta Campaign. If 21 in 1870 he was 16 in 1865, which is a strech but possible.