Custer was quite a success in the Civil War, becoming the youngest general officer in US history. Unfortunately, being reduced to a mere Lieutenant Colonel (My rank at retirement), he refused to acknowledge the reduction, wore stars on his uniform, and was a driven, vainglorious individual...a disaster, waiting to happen.
Of course, I didn’t go to West Point.
He actually was not reduced in rank. He never held a General’s Commission in the U.S. Army. During the Civil War He was a Brevet Brigadier General. This rank was temporary and held until the war ended. At the end of the war, his brevet commission expired. He was then appointed Lt Col. in the Regular Army. He was lucky, in that respect, the Union Army shrank from about 800,000 men in 1865 to a Regular Army of about 15,000 men in 1866.
He was not the youngest General in US History at that point.
That honor went to Ranald MacKenzie (USMA 1862) if you count West Point grads. And even then it was only a Brevet Rank.
Or you could say Galusha Pennypacker was the youngest. The reason nobody firmly says he’s the youngest is because there are discrepancies in his birth documents.