I question that he was the youngest general of either army.

His 1861 West Point Classmate, George Armstrong, born 1839, was appointed BGen. on 29 June 1863 at the age of 23.

Young, born in 1836, three years earlier than Custer, was promoted BGen. 28 Sept. 1863, age 26.

Data taken from Mark Boatner's "The Civil War Dictionary."

I wouldn't be suprised that there may have been a younger General officer.

Fighting Joe Wheeler, also born in 1836, West Point 59', was promoted BGen.
30 Oct. 1862.

Hugh Judson (Kill) Kilpatrick, again an 1836 product, USMA classmate of Young and Custer, was promoted Bgen. 13 June 1863.

George Martin