That varies from male to male. It depends on how fast the guy's hormones and organs become mature. As for "pre cum", and your comment about it "being enough to impregnate a woman, I think you need to know what that fluid is and how things work, so bear with me OK? : The prostate gland secretes a fluid (prostatic fluid, which you are I believe calling "pre cum") that makes up about 20-30 percent of the total volume of the "seminal fluid" that is released when a guy ejaculates. This prostate fluid is continuously made in the prostate gland, but it increases alot during sexual excitement. It seeps out of the penis head before any ejaculation ever happens, and is a sign of sexual excitement, similar to when the woman's Bartholin Glands secrete a lubricating fluid and that makes their vagina moist or even wet when she gets real turned on. The combination of sperm, seminal vesicle fluid and prostatic fluid, in addition to a tiny amount of fluid from some other minor glands, constitutes semen which make a woman oregnant if one of them fertilizes an egg. The prostate gland fluid (what you call ("pre cum" I believe) is a thin, milky substance that gives semen its color and odor. The chemical contents of these secretions (just in case you are curious) are calcium, zinc, citric acid, acid phosphatase, albumin, and prostatic specific antigen. **These substances aid in the lubrication of the urethra (the tube inside the penis which urine and cum go through), and protection, nourishment, and mobility of the sperm in the normally acid environment of the female vagina.** This is important to know, since sperm fluid is more likely to get to where it needs to go to make a woman pregnant if things are very slippery and wet, aiding the sperm to swim more easily. I know this is more than you asked, but the question needed abit more of an answer so you understand how the whole process works I think. Also to protect you from any diseases, be sure and wear a rubber/condom if you have sex at any age. Otherwise you take the chance of catching or giving real nasty and dangerous diseases to your sex partner, not to mention getting your sex partner pregnant.