For example, John and Thomas J. Walthall of Marion, Ala., appear with the class of 1864. In the 1860 census of Perry County they are reported in their father's household as ages 13 and 14. I would expect to have found them in a Confederate unit later in the war, but fail to locate anything. Two older Walthalls by the same names can be found in the Gid Nelson Battery, but not these two. Their University of Alabama profiles list later Confederate service, but I cannot identify anything.
For what it's worth, the youngest student I could find during the war classes was Benjamin Franklin Powell of Montgomery, born Sept. 29, 1850. He belonged to the class of 1865 and would've been 14 when the war ended. Powell appears on the 1860 census of Montgomery as age nine. The next youngest University cadet was born in 1849.