Death penalty has for a long time been exposing interesting trends of social discrepancies based on life determining attributes like wealth, education race.
How does the answer compare to the youngest black, or other non-white person sentenced to death in the U.S.A.?
The youngest that I can find evidence of was Irving Hanchett, executed in Florida at 15 on 5/6/1910.
Irving Hanchett, barely 15 years of age, was executed by Florida in 1910. Only three months elapsed between his crime (the murder of a teenaged girl who rejected his advances) and his execution. Hanchett had just moved to Florida from Connecticut and had no frieds or family in the area. While he was awaiting execution, a priest baptized him into the Catholic faith, and Hanchett seemed to be buoyed by the hope of salvation. As the 15-year-old mounted the scaffold, he said: "Marcy, my Jesus, my Jesus, mercy. Goodbye everyone" (Florida Times Union [Jacksonville], 7 May 1910).1
Note that there are some discrepancies as to his age, for example this newspaper article reports him as being 18:
Deland, Fla., April 8. -- Irvin Hanchett, the 18-year-old boy charged with the murder of Clevie Tedder, aged 14, on February 12, was placed on trial here today. The child's body was found near the roadside, not far from her home, with more than sixty knife wounds, made with an ordinary pocket knife.2
However, the preponderance of materials I could find gave his age at 15.
Note that this is the youngest that I can find substantiating record of. The database at deathpenaltyusa.org lists an unnamed white male executed for arson in Suffolk County, Massachusetts in 1837 at the age of 13, but I can't find any source material that would corroborate that.
EDIT:
In response to the edited question, the youngest person (and certainly the youngest female) generally believed to have been legally executed in the United States is Hannah Ocuish, executed at age 12 in Connecticut in 1786.
Note that the deathpenaltyusa.org site linked about records 2 other 12 year olds executed, both black slaves, a boy in Virginia by the name of Clem on 5/11/1787 and another boy named Bill on 7/30/1791. In the latter case, there seems to be a disagreement on the location. I'm guessing that the LA Times is the more accurate account.3
1 Radelet, Michael ed. Facing the Death Penalty: Essays on a Cruel and Unusual Punishment, p 52
2 Morning Examiner. (Bartlesville, Okla.), Vol. 15, No. 46, Ed. 1 Saturday, April 9, 1910, p 3
3 Los Angeles Times. Death for Juvenile Crimes : Execution, a Practice Dating to 1642, May Continue This Week, January 07, 1986 p online
It is certainly possible that, if one considers persons put to death by the civil and military officials of the Confederate States of America to be legally executed instead of murdered, the youngest white person executed would be a victim of the CSA during the four violent years of its existence.
As an example; Some German Americans resisted the secession of Texas and I have read that a number of men and boys were hung by the Texans because of that.
I have read a statement that a 14 year old boy in the 2nd Minnesota infantry was hung by Rebel cavalry under General Wheeler for unspecified reasons, possibly after some type of court martial.
According to Co Aitch, by Sam Watkins, two teenagers were hung as Union spies at Chattanooga in the Summer of 1863:
can now recall to memory but one circumstance that made a deep impression on my mind at the time. I heard that two spies were going to be hung on a certain day, and I went to the hanging. The scaffold was erected, two coffins were placed on the platform, the ropes were dangling from the cross beam above. I had seen men shot, and whipped, and shaved, and branded at Corinth and Tupelo, and one poor fellow named Wright shot at Shelbyville. They had all been horrid scenes to me, but they were Rebels, and like begets like. I did not know when it would be my time to be placed in the same position, you see, and "a fellow feeling makes us wondrous kind." I did not know what was in store in the future for me. Ah, there was the rub, don't you see. This shooting business wasn't a pleasant thing to think about. But Yankees—that was different. I wanted to see a Yankee spy hung. I wouldn't mind that. I would like to see him agonize. A spy; O, yes, they had hung one of our regiment at Pulaski—Sam Davis. Yes, I would see the hanging. After a while I saw a guard approach, and saw two little boys in their midst, but did not see the Yankees that I had been looking for. The two little boys were rushed upon the platform. I saw that they were handcuffed. "Are they spies?" I was appalled; I was horrified; nay, more, I was sick at heart. One was about fourteen and the other about sixteen years old, I should judge. The ropes were promptly adjusted around their necks by the provost marshal. The youngest one began to beg and cry and plead most piteously. It was horrid. The older one kicked him, and told him to stand up and show the Rebels how a Union man could die for his country. Be a man! The charges and specifications were then read. The props were knocked out and the two boys were dangling in the air. I turned off sick at heart.
Canada being but a stone's throw across the Detroit River, and even closer across the broad Prairies of the mid-west, I include from it history a more recent well known case.
Steven Truscott was convicted of the murder of his classmate Lynne Harper in 1959, when only 14 years of age, and was sentenced to death by the judge despite a jury recommendation for mercy. Upon the failure of his appeal in 1960, and in line with the de facto abolishment of the death penalty in Canada about that time, his sentence was commuted by the Government of Canada to life imprisonment.
In 2007 his conviction was overturned by the Ontario Court of Appeal upon consideration of modern forensic evidence obtained by exhuming Harper's body; though the court was unable to make a declaration of factual innocence based upon that evidence.
