This page is pretty complete with examples and references.
In summary a lot of the evidence is empirical - you look at smokers and the quality of their skin and compare to non-smokers and the differences are statistically significant and reproducible. It seems as if this is old work, dating from the 1970s.
So in many cases these are only observed correlations. But they are pretty strongly agreed upon by medical people. Given the fact that we still don't know exactly what causes cancer, I think this is not an unusual level of explanation for medical conditions.
As you can see some theories would include direct effects of the smoke and some indirect effects.
It is not certain exactly how smoking causes early ageing of the facial skin. Theories include:
- Heat from the cigarette directly burning the skin
- Changes in the elastic fibres of the skin
- Narrowing of blood vessels (vasoconstriction), which reduces blood supply to the skin and can cause changes in skin elastic fibres and
loss of collagen
- Reducing Vitamin A levels and moisture of the skin