It's never too late to quit smoking - good job!
Robert: we all lose lung function after the age of 25 - 30, but the decline in non-smokers is relatively small. Not all smokers have the same risk of developing loss of lung function, but some do- and at an alarming rate. Fortunately, we have a significant pulmonary reserve not to notice a mild loss. (Add in that most of us are fairly sedentary, and we're not really hitting the reserve point on a day to day basis).
Other things can lead to emphysema, true, but smoking is a huge risk factor. The emphysema may be "silent" - it's just loss of lung tissue, like "swiss cheese" of the lungs. The emphysema, and chronic bronchitis that may also occur (or occur independently) leads to a loss in lung function, measured as the amt of air that can be expelled in 1 second (Forced expiratory volume in one sec or FEV1). When the FEV1 declines below a certain threshold, we often get symptoms.
So- your friend may have loss some lung function while he smoked, but still had a good reserve at that time. With aging, he just continued to lose lung function until he became symptomatic.
Graphically:
and article:
http://www.aafp.org/afp/20060215/669.html
gotta go - hope this helps
-nos