While puberty begins and ends at different times for different people, puberty always begins with the appearance of a substance called gonadotropin-releasing hormone (GnRH) in the body. The release of GnRH is controlled by the hypothalamus, the same part of the brain that regulates your temperature and your sleep cycle, as well as your senses of hunger and thirst.
With that in mind, let's take a look at the different stages of puberty. While not strict guidelines, these stages provide a pretty good idea of the overall appearance and occurrence of the different changes that happen in a male's body during puberty.
Boys generally begin puberty sometime around age 10, though it isn't uncommon for it to begin as early as age 9 or as late as age 12. GnRH is released, the testicles begin to mature and an initial growth spurt may occur (with average growth being around 2 inches (5.1 centimeters) a year). Although the growth of the scrotum and testicles is one of the first outward signs of puberty, boys at this stage are still unable to reproduce. There may also be the appearance of very fine hair in the pubic area. Boys may occasionally experience erections.
The second general stage of puberty usually occurs around age 12 or 13. The testicles continue to grow, and this in turn means that more testosterone will be flowing through the body, spurring more changes. The boy will continue to grow taller rapidly, at a rate of 2 to 3 inches (5.1 to 7.6 centimeters) a year. What little pubic hair exists may begin to gain some color. Erections will become more frequent. The boy's body will begin to take on a leaner, more adult and masculine shape.