yitz17:
While it the Chofetz Chaim that makes an issue of starting smoking as being an aveiro because of the dependency, most other poskim focus purely on the medical damage. I refer you to some of the seforim on smoking that address the subject of the single cigarette, quoting the Gedolei Haposkim who are unanimous in declaring the issue involved from even this “minimal” damage. To treat this lightly is irresponsible, and not consistent with halacha.
It is true that stopping smoking can often lead to enough healing of whatever body tissues have been affected. Counting on that as a heter to start is stupid. It is like cutting yourself just because the wound will eventually heal. So the notion of bochurim being allowed to take up smoking on the smach they will later quit is immature, medically foolish, and against halacha.