The teen, who has not been identified publicly due to his age, accepted plea deal that will stay a 15-year adult sentence provided the boy does not violate the condition of his extended juvenile sentence, which will be in effect until he turns 21.
Sentencing is set for Jan. 27.
Brok Junkermeier, 19, is accused of stabbing and strangling Lila Warwick after he forced her to write him a check during the robbery, which was allegedly orchestrated by Warwick's own grandson.
According to the charges, 18-year-old Robert Warwick believed his grandmother had more than $40,000 in a safe in her Willmar home.
Both Warwick and Junkermeier were indicted on first-degree murder charges and face potential prison sentences of life without parole.
The active grandmother and great-grandmother was an active member of her church and frequently volunteered there prior to her death. Her body was found after she failed to arrive at a planned church event and then failed to pick up her 13-year-old granddaughter, who told Fox 9 News had planned to stay the night at Warwick's home the evening she was killed.
Information from the Associated Press was used in this report.