The onset of puberty is linked with the start of sperm production. Usually it starts around 13 years of age, although this can vary from boy to boy. The male testicles (testes) produce spermatozoa, more commonly known as sperm. Testes are glands within the scrotum, a sack of skin which holds the testes away from the body. This allows the scrotum to regulate the temperature of the testicles - by expanding away from, and contracting in to the body - making it an ideal environment for sperm and hormone production.
Sperm production is an ongoing process. It normally takes about 70 days for a sperm to mature. First the sperm forms in the testicles, then it travels through the epididymis till it reaches the vas deferens. The sperm is then stored until ejaculation occurs.
During ejaculation, spermatozoa and liquid from the prostate and other glands blend while passing through the urethra. The combination of prostate liquids & spermatozoa is called semen. The urethra also connects to the bladder for passing urine. During intercourse the connection is interrupted so that the semen does not come into contact with urine.
The testicles produce millions of spermatozoa each day. Spermatozoa are delicate and have a very limited chance of survival. This is why an ejaculate consists of hundreds of millions of sperm. Only a few of which will survive the journey through the female vagina. Out of the few, only one usually fertilizes an egg.
SexualityandU.ca (2009). "Reproduction: Life after puberty". The Society of Obstetricians and Gynaecologists of Canada.
Purves, et al. (1995). Life: The Science of Biology, 4th Edition. Sinauer Associates.