There were 10,000 Civil War veterans alive in 1940, with an average age of 94. The last verifiable Confederate veteran, Pleasant Crump, died in 1951 at the rather advanced age of 104, and he only lasted so long because he was 14 years old in 1861. I can't point to any one source disproving it, as it's exceedingly difficult to prove a negative, but I don't believe any Civil War veterans participated in either WWI or WWII. Joseph Wheeler, a Confederate general, did lead troops in the Spanish-American War, but he was a very old man even then, and I can find no other reference to Civil War veterans in American wars.