What is the prostate gland?

The prostate gland is a part of the male reproductive system. Therefore, women do not have a prostate. 
The prostate sits below the bladder and secretes prostatic fluid which constitutes about 25-30% of the male ejaculate. Prostatic fluid gives semen its characteristic colour and it is slightly alkaline. This alkalinity helps to protect sperm from the low pH levels in the vagina, and the prostate fluid supplies sperm with essential nutrients as well.

Normal size of the prostate

The prostate is about the size of a pea in infants. It grows very little until puberty, at which time it doubles in size, to the size of a walnut. A normal prostate typically weighs about 20 grams when fully developed.
In time, the prostate increases in size and the percentage of older men with an enlarged prostate is quite staggering.
Information on prostate enlargement