Charles Martin called 911 on Sunday afternoon, saying calmly: "I just killed a kid."
Police, who released the call's contents, said Martin also told the dispatcher: "I've been harassed by him and his parents for five years. Today just blew it up."
Larry Mugrage, whose family lived next door, was shot in the chest with a shotgun. The high school freshman was pronounced dead at a hospital.
Martin, 66, allegedly told police he had several times had problems with neighbors walking on his lawn. He remained jailed without bond Monday. His jailers said no attorney was listed for him.
Neighbors said Martin lived alone quietly, often sitting in front of his one-story home with its neat lawn, well-trimmed shrubbery and flag pole with U.S. and Navy flags flying.
Joanne Ritchie, 46, said Mugrage was known as "a good kid," but she also always considered Martin to be friendly.
Union Township is near Batavia, about 20 miles east of Cincinnati.