Puberty starts at different ages for boys and girls, and the changes happen at different rates. It's impossible to know in advance exactly when puberty will start, but for boys the changes usually start sometime between the ages of 9 and 15 years. The rate of change is also different in each individual, so your teen's voice may start to change before or after his friends', and this is perfectly normal. This isn't usually the very first stage of puberty, but you will know it's coming as your teen exhibits the first stages of puberty, some of them physical and some emotional. Generally, the voice starts to change at the same time as the penis grows, sometime between the ages of 11 and 15.
The voice changes in both boys and girls as they go through puberty, but for boys the change is more dramatic and the voice becomes much deeper. The once-childlike voice becomes that of a man as the body produces the male hormone testosterone, which causes (among other things) the larynx and vocal cords to grow. As the larynx grows bigger, it tilts and some of it sticks out so you can see it through the neck. This is commonly known as the Adam's apple; only men have an Adam's apple because their larynx is bigger, and so their voice is deeper. Children have a small larynx and thin vocal cords, and as the vocal cords get longer and thicker the voice deepens. However, as the larynx grows, your teen will experience those cracks and changes in pitch that may cause him embarrassment. This change happens in stages, and for awhile the pitch of his voice may go up and down, causing it to crack or squeak unexpectedly. When the larynx finishes growing your teen will have a deep voice that he can control.