Read this, this is what islam say about
THE BEGINNING OF SEXUAL LIFE
Sexual desire is aroused in human beings at the age of puberty. In Islamic legal definition, puberty (bulugh) is determined by one of the followings:
1. age: fifteen lunar years for boys and nine for girls;
2. internal changes:
In boys: The first nocturnal emission. Semen accumulates in the testicles from puberty onwards and more semen may be formed than the system can assimilate; when this happens, semen is expelled during the sleep. This is known as nocturnal emission and wet dream or ihtlam in Arabic. In girls: Menstruation. Right from their birth, the girls’ ovaries contains about 400,000 immature eggs; at puberty, the eggs start maturing, usually one ovum each month. If no egg is fertilized, the egg together with the lining of the womb is discarded in form of what is known as menstruation and monthly period or hayz in Arabic.
3. physical change
Growth of coarse hair on lower part of abdomen. Since sexual urge begins at puberty and as Islam says that sexual urge should be fulfilled only through marriage, therefore, it has allowed marriage as soon as the boy and the girl reach the age of puberty. In case of girls, it not only allows them to be married as soon as they become mature, but also recommends such marriage. It is based on such teachings that Islam discourages girls from postponing their marriage because of education; instead, it says that girls should get married and then continue their education if they wish to do so.