Who is the youngest officer to hold the rank of general in the United States Army?

Galusha Pennypacker (June 1, 1844 – October 1, 1916) was a Union general during the American Civil War. He is to this day the youngest person to hold the rank of brigadier general in the U.S. Army; at the age of 20, he remains the only general too young to vote for the president who appointed him.

Pennypacker's greatest moment of the war came at the Second Battle of Fort Fisher, January 15, 1865, where he was again severely wounded. His wound was considered fatal and General Terry promised the young officer that he would receive a brevet promotion for his conduct that day. Terry called Pennypacker "the real hero of Fort Fisher" and remarked that without his bravery the fort would not have been taken. He was much later awarded the Medal of Honor, with a citation reading, "Gallantly led the charge over a traverse and planted the colors of one of his regiments thereon, was severely wounded."

He received a brevet promotion to brigadier general dated January 15, 1865. He survived his wounds after 10 months in the hospital and on April 28, 1865, he received a full promotion to brigadier general of volunteers at age 20 (backdated to February 18), making him the youngest officer to hold the rank of general to this day in the United States Army (though Marquis de LaFayette was just 19 when he received his Major General's commission in the Continental Army on 31 July 1777). He was appointed a brevet major general of volunteers on March 13, 1865.