For age at a given time use a civil or personal record like the census, family Bible or gravestone. In 1850 this man gave his age as 32; in 1860 it was reported as 45. Most family researchers use 1818 as his birthyear. In that case WBB would not have been 52 until 1870.
The 1860 census reports the Barneycastle household as located near Nathansville (near present-day Riverview). That's less than five miles from the Florida line in Escambia County. A number of men who joined this man's Florida company lived in what was then Conecuh County AL.
According to the law passed by Congress on Apr 16, 1862, nearly all white male citizens between the ages of 18 and 35 were required to be in Confederate military service. The law allowed substitution, which meant that a man could put someone in the army to fill his place. The substitute had to be someone not required to serve. WBB fit the bill by virtue of his age. He was still overage when Congress raised the upper age range from 35 to 40.
At the end of 1863 Congress abolished substitution, which meant that both the substitute and his "principal" were required to serve. On Feb 17, 1864, Congress passed another law which placed men ages 45 to 50 in something called senior reserves. More than a few people were confused as to what they were obligated to do at this time, and the Confederate war effort was going badly. It seems that WBB deserted at some point in 1864.
Anyone who provided his own horse for Confederate cavalry service received 40 cents a day. Depending on how many days there were in the pay period, payment for use of horse could vary.
Please feel free to respond with any questions.