Sperm production, called spermatogenesis, begins at puberty, usually around age 12. Once sperm production starts, it will typically continue uninterrupted until death, though discernible defects in sperm quality and sperm quantity increase with age.
A healthy male reproductive system will produce around 50–100 million viable sperm each day. The whole process of spermatogenesis, from newly formed sperm to a fully mature one, usually takes around 74 days.
Spermatogenesis takes place in the testes, starting in vessels called “seminiferous tubules.” There, a germ cell (a “diploid” cell with 46 chromosomes) divides three times to produce multiple haploid spermatids (immature sperm). These divisions of the sperm cells create genetic variation. At this point, the sperm cells that have divided are still connected with cytoplasm.
In the next step, the spermatids begin to mature, a process driven by the male hormone testosterone. They form a tail and an acrosome, their DNA becomes more compact, and excess cytoplasm is removed to create individual structures.
At this point, sperm are still immotile (unable to move on their own). They are transported via muscle contractions to the epididymis, a coiled tube behind each testicle, where they mature further, gaining the ability to move on their own and fertilize an egg.
Relatively cool temperatures are important for spermatogenesis. That’s why, during puberty, the testicles descend from the body to hang between the legs. This keeps them about 4ºF cooler than the usual body temperature of 98.6ºF.