There is not an exact age for what age boys voices deepen, because we all go through puberty at a different time. Approximately, boys voices deepen around eleven through around fourteen. That still doesn't mean their voice is changed and is not going to still change, because it could still get deeper. Also, a boy's voice can change sooner or even later. I know this because there are a lot of boys in my grade (Sixth grade) I've noticed that their voice deepened.