Smoking Age of 18!

What's the point of increasing the age to 21?

The legal age of smoking is 18 in New York at the moment, but, the Health Commissioner has been trying to pass a law that raises the age to 21. What difference will it make exactly? Those who are already 18+ will be allowed to smoke under the ban but won't be able to buy it. Their friends, family, etc. that are of age could easily purchase cigarettes for them. Also, underage people already smoke illegally so, what will stop them from continuing?

People who are legal adults should be able to make their own decisions.