Life-long smokers often ask their doctors if it is worthwhile quitting at an older age and if it will really make a difference in their lung function and overall survival.
In fact, it does make a difference! We all loose lung function as we age and smokers can loose lung function more quickly. When smokers stop, the rate of deterioration goes back to the normal rate.  
3. Anthonisen NR, Skeans MA, Wise RA, Manfreda J, Kanner RE, Connett JE; Lung Health Study Research Group. The effects of a smoking cessation intervention on 14.5-year mortality: a randomized clinical trial. Ann Intern Med. 2005;142(4):233-9.