I think Jacklyn Lucas still holds the record.
http://en.wikipedia.org/wiki/Jacklyn_Lucas
Private First Class Jacklyn Harold Lucas (14 February 1928 – 5 June 2008) was a United States Marine who was awarded the Medal of Honor for his heroic actions during the Iwo Jima campaign — for unhesitatingly hurling himself over his comrades upon one grenade and for pulling another one under himself. One of the grenades exploded, and Lucas absorbed the entire blasting force of it with his own body. Private First Class Lucas, the youngest Marine ever to receive the United States’ highest military decoration, was presented the award by President Harry S. Truman at the White House on Friday, 5 October 1945.
Although only 14 years of age, having a muscular build, 5 ft 8 in (1.73 m) tall and weighing 180 pounds (82 kg),[2] he enlisted in the Marine Corps Reserve without his mother’s consent[2] on 6 August 1942. He gave his age as 17, and went to Marine Corps Recruit Depot Parris Island, South Carolina for recruit training.
During his rifle training, Pvt. Lucas qualified as a sharpshooter. He was next assigned to the Marine Barracks and Naval Air Station Jacksonville, Florida. In June 1943, he was transferred to the 21st Replacement Battalion at New River, North Carolina, and one month later he went to the 25th Replacement Battalion, where he successfully completed schooling which qualified him as a heavy machine gun crewman.
He left the continental United States on 4 November 1943, and the following month he joined the 6th Base Depot of the V Amphibious Corps at Pearl Harbor, Hawaii. He was advanced to private first class on 29 January 1944.
With statements to his buddies that he was going to join a combat organization, PFC Lucas walked out of camp on 10 January 1945, wearing a khaki uniform and carrying his dungarees and field shoes in a roll under his arm.
He was declared UA (Unauthorized Absence) when he failed to return that night and a month later, when there was still no sign of him, he was declared a “deserter”, and a reward was offered for his apprehension. He was also reduced to the rank of private at that time.
He stowed away on board USS Deuel which was transporting units of the 5th Marine Division into combat. He surrendered to the senior troop officer present on 8 February 1945 dressed in neat, clean dungarees. He was allowed to remain, and shortly after he was transferred to Headquarters Company, 5th Marine Division. He reached his 17th birthday while at sea, six days before the heroic actions at Iwo Jima, for which he was awarded the Medal of Honor.
On the day following the landing at Iwo Jima, he was creeping through a twisting ravine with three other men of his rifle team when the Japanese opened an attack on them. The men were in a trench when two enemy grenades landed nearby. Lucas pushed a thrown hand grenade into the volcanic ash and covered it with his rifle and his body. He reached out and pulled a second grenade beneath him. He was left for dead by his companions, although he was miraculously still alive. Severely wounded in the right arm and wrist, right leg and thigh, and chest, Pvt. Lucas had undoubtedly saved his companions from serious injury and possible death.[1]
He was evacuated to the hospital ship Samaritan, and then treated at various field hospitals prior to his arrival in San Francisco, California on 28 March 1945. He eventually underwent 21 surgeries. For the rest of his life, there remained about 200 pieces of metal, some the size of 22 caliber bullets, in Lucas’ body — which set off airport metal detectors.[3]
The mark of desertion was removed from his record in August of that year while he was a patient at the U.S. Naval Hospital at Charleston, South Carolina. He was discharged from the Marine Corps Reserve because of disability resulting from his wounds on 18 September 1945, following his reappointment to the rank of Private First Class.
On 5 October 1945, Lucas and 14 other sailors and Marines (including Pappy Boyington) were presented the Medal of Honor by President Harry S. Truman. In attendance at the ceremony were Lucas’ mother, Admiral Chester Nimitz, and Secretary of Defense James Forrestal.[3]