I like the idea of smoking; people who do it seem to get a lot of pleasure from it. But I don't want it seriously to curtail my longevity. So at what age could I sensibly start smoking in order to achieve sufficient pleasure to make the shortening of lifespan worthwhile?
You're right to point out that smoking has benefits as well as costs, and that implies that it might be perfectly rational to smoke. You also implicitly accept that smoking is addictive, since your cigarette consumption plan seems to be to start and never stop. But this consumption plan may be poorly timed. For one thing, there is some evidence that if you smoke while young and can quit before you die of a heart attack, your risk of heart disease rapidly recedes. (Don't ask me about the cancer risk, though. I'm an economist, not an epidemiologist.)
Leaving aside the medical questions, there's the social side. If you're 20 years old and smoke in full defiance of the risks, you still have a chance to look brash, daring and just a little bit sexy. If people see you smoking at 60, they won't realise your brilliance in taking up the habit at age 58. They'll simply assume that, pathetically, you've never been able to quit.
My advice, then, is that the optimal consumption path for cigarettes is either never to start, or to start young and stop fairly quickly. Which you choose depends on, among other things, your estimate of your own will-power. This is hard to guess in advance. Perhaps you should tell your doctor you plan to start smoking, and ask him for advice about how to quit once you've started. At least you'll get his attention.