Generally speaking you're correct, however, if possible you need to identify the document from which it was taken. It could be date of discharge, parole, death, etc.

There has to have been an error transcribing his data by the War Department clerks who produced these Compiled Service Records. Apparently neither age is correct.

Your best evidence would be either his word, or census information.

He is recorded on the Historic Roll for the regiment as age 26 when enlisted, born in Platte, Missouri, Farmer, resident of Platte City, Platte County, was at Vicksburg, Miss., deserted at Demopolis, Ala. July 24, 1863

M322: Compiled Service Records of Confederate Soldiers Who Served in Organizations from the State of Missouri