This is the classic example of public welfare versus personal liberty. If we accept that for legal purposes, the age of majority is 18, then this is the age in which persons are considered competent to enter into contracts, be charged as adults for crimes, etc., and so it is a logical choice for purchasing cigarettes; perhaps some of the discussion should address whether 18 is still the appropriate age to consider the age of majority. The age of majority varies between 15-21 throughout the world.
If a person makes a well-informed decision to take actions that harm himself/herself, I do not think it is the role of public policy to prohibit such, even if indirectly harms the collective (through increased medical expenses, etc.), on the principle of self-determination. Because cigarettes provide no real benefit, and can have significant costs, a cost-benefit analysis would say cigarettes should not be used by anyone, and a person could argue for a universal prohibition on such grounds. However, it is my opinion that the cost-benefit analysis is up to each individual, and the role of public policy is to ensure that individuals are appropriately informed of the risks that one faces.
Some people never mature enough to make important decisions appropriately; some people are mature enough at 13 to sensibly decide the cost-benefits of such decisions. The age of majority is, by definition, an arbitrary approximation of an age in which a person is ready to make such decisions. I would argue that it is good public policy to be consistent, even if it is at an arbitrary age. For that reason, I believe purchasing tobacco products should be limited to those 18 years of age.