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.
I suppose most boys can first ejaculate around age 13-14. Early bloomers, who start the puberty process earlier than others, reach this stage sooner, while late bloomers need to wait longer for their bodies to reach that point in the process, before they can actually ejaculate. Another factor that affect ejaculation volume is the frequency.