The main reason is addiction brought about by cultural norms.

Parents discourage children from doing something or behaving in certain way as part of their upbringing.
We may, for instance, discourage a child of 3 from drinking tea or coffee but may allow the child to do so upon the child turning 5 or 6.

Likewise, with drinking alcohol and smoking although in these cases the parents wait until their children are 16 - 21 depending on the country and culture. In all these cases allowing the children to do something which was discouraged when they were younger, makes them feel grown up and happy.

The cultural norm to discourage children from doing something until grown up to a certain age, creates curiosity and a desire to grow up. Because of the curiosity and the desire to be grown up, when they reach the age of consent, they are driven to experiment with these behaviors and actions, although they don't have to and in most cases develop an addiction to the thing that was once denied to them.