It is usually about 24 months after the beginning of pubic hair growth when a boy has his first ejaculation of seminal fluid (either as a wet dream or as a result of masturbation).
When a male ejaculates, his muscles contract and sperm and semen mix and are pushed into the urethra and come spurting out of the opening in the glans — or tip — of the penis. The semen usually comes out in three or four spurts. In all, about a teaspoon of white, creamy, milky semen comes out of the penis during ejaculation.
Ejaculation after masturbation is important. Stopping the ejaculation, by covering the end of the penis with a finger or thumb, can cause "retrograde ejaculation," which means the semen travels back up the urethra and can be forced up the tube that leads to the bladder, or forced into the prostate gland.
In either case, there may be pain and discharge from the penis. If this happens, you should see your doctor.