Anyone old enough to be sent to a foreign land to die should be able to use tobacco products if he or she chooses to do so. Many things in life are not especially healthy, but we live with constant trade offs between health and self satisfaction and gratification. Tobacco is one such choice adults, including some teens, get to make.
No, it should not be legal for teenagers to use tobacco products, because tobacco is very damaging for a person's health, and it is very addictive. A teenager is too young to decide whether they ultimately want to die of lung cancer, or whether they want to have an expensive addiction for the rest of their lives.