The Atlanta Journal-Constitution reported that it took the Douglas County jury 30 minutes to make the determination Wednesday.
Gevin Allen Prince remains jailed. He was charged as an adult in the August 2011 stabbing death of 77-year-old Mary Joan Gibbs.
Authorities say the teen also injured his grandmother, 55-year-old Laura Prince, who recovered from her wounds. Authorities say the teen attacked the women after he was told to stop playing video games.
A trial date has not yet been set.