ANDY'S ANDSWER: When a boy starts puberty, the hormones in his body start the production of sperm. One of the first physical signs is that his testes will start to grow larger. He may notice sperm in his morning urine, or later on have nocturnal emissions - semen that comes out at night while asleep, often called a wet dream. The testes continue to grow throughout puberty, and it takes a year or more to have full production of sperm. As the testicles are growing, the scrotal sac will begin to hang to accommodate the sperm and to keep it cooler.
On average, boys begin puberty at around 13, but it is normal to begin at an earlier or later age as well.