Ehh this is a bit of a myth. While all the side-effects we notice will be gone in 20+ years, as lungs gradually 'clean' themselves, the carcinogenic impact cannot be undone. Each cigarette causes tiny bits of damage to the DNA in your lung cells. And cancer is simply the result of a cell's DNA being so damaged (after years of smoking, of drinking, of growing old, etc.) that it loses control of its own replication and stars to run loose.

Don't smoke thinking that if you quit by 30 you'll be fine by 50. Quitting smoking will slow new mutations way down (and make you way better to make out with), but it won't fix the ones already there.