As I said earlier, some processes, such as cancer, are not easy to reverse once started. So, if someone smokes daily and for many years, his or her risks of cancer and lung disease and heart disease go up accordingly. For any smoker, be they an adolescent or an adult, the short-term effects are the same. They get the immediate effect of the nicotine and also the short-term effects of toxic chemicals in smoke. So, the short-term effects are the same in adolescents and adults.
Carbon monoxide reduces the amount of oxygen available to the brain.
Carcinogens begin to damage tissues, etc. When someone succeeds in quitting smoking, they have often tried to quit on several occasions. If you measure the time from when they first tried to quit to when they succeeded, the process can take years. On the other hand, there are many examples of persons who have made the decision to quit smoking, taken action, and have quit on their first attempt. If you attempt quitting, but are unsuccessful at first, it's important to try again using experience from prior attempts to help you.