How old can sex, there is no absolute time limit, human body development and thinking development is immature case, not recommended to have sex, generally recommended 18 years of age after the adult sex. Under the age of 18, the development of the human body is not yet complete, thinking is not mature, not enough knowledge of sex, this time premature sex, in the case of contraceptive measures are not complete, easy to cause accidental pregnancy, the growth and development of men and women have an impact. Before the age of 18, most of the people are still in the underage stage, do not know enough about sex, can not take on social responsibility, premature sex, on the school and life caused by a greater impact. In daily life, pay attention to protect yourself, understand the understanding of sex, face life positively and be optimistic.