vocal                                                                                                                                                                                                       resonating
VIBRATORY SYSTEM

As you know, the sound is always produced by something vibrating.

The Larynx (or "voice box") is the origin of sound production and acts as
the Vibrator of the Vocal Instrument.

The Vocal Cords (or "vocal folds") are inside the voice box and vibrate when air passes through them producing sound.
 They open for respiration and close for phonation (talk or sing) and to protect the air-way.
Loose vocal cords produce low sounds and tight vocal cords produce high sounds.
vibratory folds            vocal_cords    
        Vocal Cords vibrating 


Guess what?
The vocal cords are muscles and during the course of puberty, they are growing thicker.
And in the long run, they'll almost double in length!
Like guitar strings, vocal cords vibrate when contracted as air passes them.
What sound they make is determined by both how thick the vocal cords are and how much they have been contracted.
But, in general, the thicker the muscle, the lower the tones created when they vibrate.
In other words, the thicker the vocal cords, the lower the voice.

Before puberty and the change of voice, children voice is called Treble.
Boys' vocal cords grow more than girls' and changes
  in the pitch of their voices are bigger than in girls' voices.