The size of your voice box and your vocal cords affects how deep your voice is. In his early teens, a boy's voice gets deeper or "breaks". This is because his vocal cords grow twice as fast as a girl's. When you are born, your vocal cords are about 6mm (0.2in) long. They grow to 20mm (0.8in) in adult women and 30mm (1.2in) in adult men.