Just as on the demand side, the position of the supply can shift, say from a change in the price of a productive input or a technical improvement. The Law of Supply states that, in general, a rise in price leads to an expansion in supply and a fall in price leads to a contraction in supply.
Puberty takes place over a number of years, and the age at which it starts and ends varies widely. It generally begins somewhere between the ages of 7 and 13 for girls, and somewhere between the ages of 9 and 15 for guys, although it can be earlier or later for some people.
Boys tend to show the first physical changes of puberty between the ages of 10 and 16. They tend to grow most quickly between ages 12 and 15. The growth spurt of boys is, on average, about 2 years later than that of girls. By age 16, most boys have stopped growing, but their muscles will continue to develop.