On Jan. 10, James Terry Roach is scheduled to be killed by the state of South Carolina. Roach was convicted of homicide in late 1977. He was sentenced to die and has been a death- row inmate in a Columbia, S.C., prison since.

Roach's case would be slipping by as one more muffled note in the executioner's song except for his age at the time he is said to have murdered. He was 17. Of the 50 people who have been killed by state governments since 1977, only one had committed his crime under the age of 18. He was killed last September in a Texas prison. Until then, no juvenile offender since 1960 had been executed in the United States. Until now, killing adult criminals has been the American way but not the killing of people who committed crimes as children.

Roach's case is being noticed for another reason. He has appealed to the Inter-American Commission on Human Rights, a group within the Organization of American States. Roach's petition has been filed by two American attorneys and is supported by the American Civil Liberties Union, Amnesty International and other groups.

By going to the OAS commission, Roach is making an appeal to international law with a hope of success that he has not won from domestic law. Both the judicial system of South Carolina and the federal court system, including the Supreme Court, have refused to overturn his death sentence. Roach's petitioning lawyers -- Prof. David Weissbrodt of the University of Minnesota and Mary McClymont of the ACLU Prison Project -- agree that appealing to the OAS commission is not the standard approach. But it is firmly grounded in the law.

The United States is a member of the OAS. It is subject to the jurisdiction of the human-rights commission. Through his lawyers, Roach is saying that his rights are being violated because the United States is ignoring the statutes against the death penalty held by the Inter-American Commission on Human Rights. On Dec. 12, the OAS cabled both Gov. Richard Riley of South Carolina and Secretary of State George Shultz asking that the killing of Roach be stayed while the commission studies and decides on the petition. It asked for a stay "for humanitarian reasons and to avoid irreparable damages." In 1977 at the human- rights convention of the OAS, member nations agreed that the death penalty should not be reinstated in countries where it had been abolished. The United States signed.

Among the nations of the world, including those in the OAS, the execution of child criminals is rare. Forty- one nations, including the Soviet Union and Poland, forbid it. Executing criminals of any age does not occur in any of the Western industrial nations, except for the United States. As of August 1985, inmates awaiting death in American bastilles totaled 1,540. Some 250 people are added to the death rows every year. If the killing continues, the toll will soon be greater than the peak years between 1930 and 1967, when 3,859 people were executed in this country.

In South Carolina, only clemency from the governor can save James Terry Roach. The state legislature has a bill before it to prohibit executions of youthful criminals. It cannot be passed before Jan. 10. Another reason exists for extending mercy to Roach. He is anything but the maniacal killer that is the public image of the condemned.

At the time of his involvement in the killing of two juveniles -- 14 and 15 -- his worst previous offense was stealing his father's car to visit a brother in Florida. Raised in a rural South Carolina town, Roach was borderline mentally retarded. At his trial, his IQ was reported to be between 68 and 76. He is genetically at risk to Huntington's Chorea, an incurable neurological disease.

Michael Farrell, a Philadelphia attorney who is helping Roach without charge and was involved in the case before the appeal to the OAS, reports that throughout the trial and appeals it was never established that Roach pulled the trigger that killed the two teen-agers. He was one of three involved in the violence at the crime. The oldest of the trio, who was 24, has already been executed. The youngest, a 15-year-old at the time of the murders, testified for the state and is serving a life sentence.

No purpose is served in killing any criminal, least of all a retarded juvenile. In a new book, "Who Is the Prisoner," George Lundy, S.J., the director of the Institute of Human Relations at Loyola University in New Orleans, writes that punishment should accomplish four ends: restore what was lost to the victim, provide rehabilitation, apply equally to all classes of people and be no harsher than necessary. As a punishment, death fails on each.

For James Roach, as well as the 35 others facing execution for crimes committed under age 18, there is a fifth failure: the state seeking vengeance on criminals who were children, the weakest of all.