A, there are more than 40 kinds of carcinogens in the smoke produced by smoking. Smoking accelerates blood coagulation and easily causes cardiovascular diseases such as myocardial infarction, stroke and myocardial hypoxia. Smoking is an important cause of respiratory diseases such as emphysema. So, it is correct.

B, smoking first harms people's respiratory system, makes people's immunity decline, and smokers are more susceptible to infectious diseases such as tuberculosis and pneumonia. So, it is correct.

C, smoking is harmful to health, teenagers should refuse to smoke, and don't think that smoking is a mature performance, which is wrong.

D. As can be seen from the above, smoking seriously harms people's health and is an unhealthy lifestyle. So, it is correct.

So choose: C.