13 years old
While searching for some information about the Woman's Relief Corps on the Internet I found a reference to Thomas Hubler of Warsaw, South Carolina, who is reputed to be the youngest soldier to have served in the Civil War at age 13 as a drummer boy.

Well the Manning area can also lay claim to this record and below is the information.

Selden E. Whitcher
(Taken from the 1982 Aspinwall Centennial history book)
S.E. Whitcher resided on a farm in section 34, Iowa Township, a half mile from Botna, IA. At the age of 13 he entered the army as a drummer boy, serving the northern forces in the Civil War battle of Gettysburg. He later enlisted with the Army of the Potomac, and his regiment was sent west after the Civil War.

He served in the Apache War in Arizona, and he still has wounds that were received from the Indian's arrows. For a while he was a mounted scout and was on duty 480 miles from any railroad.
After retirement from Government service, Whitcher went to California and was engaged in staging for 14 years. In 1880 he came to Iowa and settled on wild prairie land. He now owns 160 acres with a nice cottage home, grove and orchard. He married Alice Moon and they have eight children, Emma, Mary, Flora, Edna, Willie, Gracie, John and Ruby. --END--

The family tradition continued through Seldon's grandson, Ruel Whitcher. Ruel attended Manning Schools into his senior year in 1929 before enlisting in the Navy. This 1944 Manning Monitor article gives some information about Ruel.

A Submarine Hero: Lt. Don Musfeldt, located in the South Pacific writes a story home about a Manning boy from who you would never hear deeds of heroism directly. Don received his information from mates of this man and passes the following information to home folks.
His letter reads in part: A couple of days ago on pass I was going to see if I could find Ruel Whitcher and I did. I just want to say a few words about him. He is now an Ensign, the same as a 2nd Lt. in the Army. He has 15 years on Submarines, an expert on them in fact. I was with him from 4 in the afternoon till midnight and really had a time. Some of his friends told me of him:
'You'll never read of him in "Look" or "Life" or anything like that, but they say he saved Subs from going down a time or two and is known as one of the best in the entire Pacific Area. His character is excellent, very modest. You'll never hear much from him-he just isn't that type.
Knowing what Ruel has done and is doing I think he is one of the most outstanding men in the service from Iowa. That's not saying too much either. He was down to my house and we are getting together as often as we can. He turned down one commission a year ago, because he wanted to be one of the fellows, just like the other enlisted men on board.
I can't tell you what he does, what ship he is on or the like, but that boy is and has done so much more than anyone I 'know of, you just can't believe it. "Oh yes, he and the fellows he stays with have a drawer full of medals, top medals one could earn. They don't say much about it. Ruel is just 32 years old and has only 4 more years to go before retirement. "He told me of numerous times they dove to miss being fired on, laying in Japanese waters for weeks on end.