A man's sperm is a living organism with a life cycle of its own. It remains living and mobile outside of the man's body and it can impregnate a woman long after the sexual contact was made. A boy's body begins to produce sperm at the time of puberty at around twelve to thirteen years of age. This is around the same time that he begins to experience "wet dreams." These dreams are the body's way of priming the boy's plumbing for use as he grows. At the time of puberty, the boy's body is producing an average of 85,000,000 sperm every day. He will continue to produce sperm throughout his lifetime in contrast to a woman whose egg production begins to taper off in her thirties.
By the time a boy becomes sexually active, he should become familiar with the way his sperm works. The most important thing to understand is that sperm is mobile and if ejaculation occurs any where near the vicinity of a vagina, pregnancy can occur since it only takes one sperm to make a baby. The fluid that the sperm lives in is what allows this mobility because it acts as a lake or a river the sperm uses to get from point A to point B. It only takes a very thin layer of this river, also known as semen or cum, to make the sperm completely mobile. Once the sperm leaves the man's body, it can live for several hours in the open but once it is inside the woman's reproductive tract, its life is extended by up to seven days.
It is important to dispel a prevalent myth, when a man masturbates, he does not "run out of sperm." A man's body makes more sperm than he could ever use in one lifetime, and there is plenty left over for several ejaculations in one day including the next time he has sex.