For the first 20 years or so of life, the growth rate is approximately 0.7 mm per year for males, and 0.4 mm per year for females, which results in a maximum adult length of about 16 mm for men, 10 mm for women. This growth of the vocal folds causes Fo to drop as predicted by the equation below:
Since children have smaller lungs and smaller vocal folds, we might expect children to be quieter than adults, but as anyone who has heard a screaming baby knows, this isn't the case. Children can produce sounds as loud as adults despite their smaller apparatus, because higher Fo's guarantee higher intensity. Also, lung pressure is 50-60% higher for children than adults, so children also make up for their smaller size by working harder vocally.
Choir directors of middle-school aged boys would likely appreciate specific guidelines regarding the vocal changes of their groups' members. However, because voice changes in pubescent males can vary widely between young men, only general statements may be made. A group of seventh-grade boys, for example, would probably represent all stages of voice maturation. Adolescent boys' voices often begin to mutate at 12-13 years of age and taper between 15-18. During this time, lower pitches tend to be more stable than upper pitch ranges, and most of the active changes tend to occur within one year.
Female pubertal voice changes are often less obvious than that of males. However, adolescent girls' voices tend to exhibit increased breathiness or huskiness, occasional "cracking", a lowering of average speaking fundamental frequency, and increased pitch inaccuracy while singing. The physiologic components that account for voice changes include facial development (related to voice resonance), a descent of the larynx (effectively lengthening the vocal tract), and increased circumference of chest wall and lung (providing greater breathing capacity).
Adulthood and Advanced Age
Although the voice itself remains stable, physiological changes do occur in middle age, most significantly ossification (hardening) of the laryngeal cartilages. In some individuals, these changes can actually improve the singing voice, since a more bony support framework in the larynx better supports the tension in the vocal folds. If you compare the voice to a piano, for instance, the strings in a piano are attached to solidly-anchored metal posts at each end. This allows the piano strings to stay in tune and make a predictable sound. At younger ages when flexible cartilage supports the vocal folds, there is a greater chance for unpredictability, but with stiffer, more bony supports, it is logical that the voice could perform more reliably.
Other changes in middle age can be less beneficial to the voice. These age-related changes affect soft tissues:
Some cells become dystrophic with age and no longer perform their normal tasks. Muscle fibers lose their ability to obey nerve impulses telling them to contract. Nerve cells lose the ability to transmit neural signals properly. As a result, the voice may become weak or fluttery.
Finally, edema or swelling of the vocal fold cover interferes with the normal vibration of the folds, which can lower the voice, or cause roughness.