The twin on the right is a smoker; the twin on the left is a nonsmoker. Notice differences in nasolabial creases
I recently read about a study conducted by the American Society of Plastic Surgeons in which they focused on identical twins who differed in smoking habits. This means that one twin smoked while the other did not. Although unsurprisingly they concluded that smoking affects the quality of the skin, the pictures are enough to show these differences.