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.
Girls hit their maximum growth spurt before menstruation begins, generally getting their first period around 12 years of age. Once their period starts, girls typically gain another 1 to 2 inches in height, reaching their full adult height by age 14 or 15. However there is a very broad range of time in which girls hit puberty related growth spurts.