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.
A growth spurt normally lasts for two years and starts at different times for girls and boys. For girls, it typically starts around age 9 to 10. The fastest point is around 11 to 12 for girls. Boys usually start around age 11 and peaking at 13. During this time, boys typically grow about 4 inches each year.