The question of whether or not smoking tobacco is sinful has been debated on the forums before, but I hope that an apologist can supply a solid answer instead of opinions on this matter. Is smoking tobacco sinful or not?
In and of itself, the action of smoking is morally neutral. The potential dangers of the particular product – in this case, tobacco – and the just restrictions on the activity by the civil authority determine whether or not it is morally legitimate to smoke it. Taking the latter consideration first, certainly it is not morally legitimate for anyone below the legal age to smoke tobacco because setting a minimum age requirement is a just exercise of civil authorities. If a young person did so anyway with sufficient knowledge and consent, the action would be sinful.
As to the potential dangers of the product: The Catechism of the Catholic Church does not forbid using tobacco; it only states that Christians should avoid using it to excess:
The virtue of temperance disposes us to avoid every kind of excess: the abuse of food, alcohol, tobacco, or medicine. Those incur grave guilt who, by drunkenness or a love of speed, endanger their own and others’ safety on the road, at sea, or in the air
(CCC 2290; emphasis in original).
Christians are called to do what is reasonably necessary to protect their health (cf. CCC 2288). If using a particular product has been proven to be a danger to health and a person disregards known health warnings and uses it anyway with sufficient knowledge and consent, it may be sinful for him to do so. Whether it is sinful or not depends on many factors, which include among others: personal moderation, the reliability of the science undergirding the health warnings; knowledge of the warnings; and consent to the action, which may be compromised by habits acquired before reliable scientific studies were made public or by habits acquired before personal knowledge of the health risks became known to the individual. Those smokers uncertain of personal culpability should consult with their confessor.
In short, all that can really be said for certain is that those adults who are considering taking up smoking tobacco should educate themselves on the health risks associated with the practice and use their informed prudential judgment to make a decision that is in line with their obligation to protect their health. If their smoking may affect the health of others (e.g., family, unborn children, nearby non-smokers) that factor should also be carefully considered.