Yes your life expectancy is decreased if you smoke, either tobacco or drugs like cannabis.
The reasons are that smoke does you damage, both to your mouth and throat as you breathe it in, and also to your lungs. There are some really nasty chemicals in tobacco as well as the nicotine, including tar, and when they burn they release nasty gasses, including corbon monoxide (the thing that faulty boilers release that can be deadly!)
These things combine to make tobacco and cannabis smoke carcinogenic, that means ‘cancer causing’. Cancers of the mouth, throat and lungs are common in smokers, and this is why your life expectancy is decreased if you smoke.
As Simon says it can also lead to heart disease as well. Basically, it’s not good!
The good news is if you stop smoking, the bad effects of smoking will disappear after a while (a good few years), but it’s still better not to start.