This doesn't really answer your question, but I'd like to put up for your consideration the oldest West Point graduate to serve in the Confederate Army, Col. Lewis G. DeRussy of the 2nd La. Infantry. He was born in September, 1795, and served as a regimental commander of the 2nd La. until July 1861. He last known active duty service was in early 1863.
So, the question now is, can anyone come up with a Confederate soldier over 68 years old?