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.
As the teenagers approach puberty, their growth usually stops. Boys tend to reach puberty by the time they turn 17 while girls do the same when they are around 15 to 17 years old. The bones of teenagers grow extremely quickly during a growth spurt, but the tendons and muscles do not.