Attorneys on Monday announced the plea agreement for 16-year-old Seth Yates of San Antonio, who was certified to stand trial as an adult.
Yates, who originally said he was fighting off a molester, pleaded guilty to murder.
Yates had claimed self-defense in last April's fatal stabbing of 59-year-old George Izer. Investigators located Yates by following a trail of blood from the victim's apartment to the teen's nearby home.
Prosecutors say video surveillance from a gas station showed Yates and Izer meeting and the teen appeared to willingly leave with the man.