Lots of smokers, especially younger smokers, rationalize their habit by telling themselves that they'll stop if and when any hint of adverse health effects develop.
The problem is that, even in the first decade of smoking, dramatic and profound effects can develop--but you won't know it.
One of the most graphic examples of this I see every day in people who have heart scans. While CT heart scans are, of course, for identification of coronary plaque/coronary disease, they're also great for visualizing the lungs.
This man is a light smoker. The lungs are the black tissues (that's normal) on either side of the (white) heart in the center. Now, note the holes in the lung tissue. That's what they literally are: holes left by the destrucive, tissue-eating effects of cigarette smoking.
How common are the holes (or emphysematous "blebs", as they're called in medical lingo)? Very common. You'll even see them in 30-somethings who've smoked only a few years.
These are holes that have nothing in them. The lung tissue that was destroyed to create the hole will never grow back, even when smoking stops. The holes in this example are actually small to average in size. I've seen much bigger. And this only represents the early stages of lung tissue destruction. A long-time heavy smoker shows all other sorts of abnormalities.
Whenever I show these "black holes" to people who smoke, they are horrified and I've actually gotten many people to quit. Take the opportunity to quit as soon as you can if you smoke.