ELIZABETH, N.J. — By all other measures it would be a routine case of the “C.S.I.” age: a detective working a cold case alights on a stray clue, and running a DNA sample, finds a man he believes is the killer. Thirty-six years after the crime, the man is brought to trial and convicted of murder.

But this 52-year-old man, now soft around the middle and wrinkled along the brow, was 15 at the time of the murder. And so, in a twist the judge called “extraordinary,” Carlton Franklin was tried as a juvenile.

His case was heard here this month in family, not criminal, court. And Thursday, after Judge Robert Kirsch found that he had killed Lena Triano — bludgeoning, raping, and stabbing — Mr. Franklin was not technically declared guilty, but rather, as “adjudicated delinquent.”

Now comes the tricky issue of how to punish him, which the judge will decide in January: Will he be sentenced according to a juvenile code that emphasizes rehabilitation over punishment, focusing on “wholesome mental and physical development”? Or will the judge look at Mr. Franklin’s life in the intervening decades, which have included 17 years in prison, but also a steady job and no criminal record in the 14 years since he was released?