1 Where one twin smoked while the other had never ever smoked.
2 and where both twins smoked but one of the twins had smoked for 5 year more than the other.

Their pictures were taken by professional photographers and the judges compared their faces for forehead features, periorbital aging and the presence of malar bags.

Image source: www.gawkerassets.com The twin on the left has smoked 17 years longer than the twin on the right. Note the differences in lower lid bags and upper and lower lip wrinkles.

Image source: www.gawkerassets.com Both twins are smokers. The twin on the right smoked 14 years longer than his brother.

Image source: www.gawkerassets.com The twin on the right is a smoker; the twin on the left is a nonsmoker. Notice differences in nasolabial creases [i.e. the lines running from the sides of the nose to the sides of the lips].

Image source: www.gawkerassets.com The twin on the left is a nonsmoker and the twin on the right smoked for 29 years. Note the differences in periorbital [i.e. the eyelid or skin around the eye] aging.

The results showed that no significant difference was there in their weights or use of alcohol, sunscreen or work stress. The smoking twin scored worse for lower lid bags, nasolabial bags, malar bags, eyelid skin redundancy, jowls, lower lip vermillion lines and upper lip lines. Their faces also confirmed a long held belief that smoking leads to faster aging.

The twin who smoked in the set with the smoking and non-smoking twin, looked older by 57%. In the second set, the twin who had smoked for longer appeared older 63% of the time. This means a 5 year difference could cause such a difference in facial aging.

Smoking does reduce collagen formation and degrades it. It also results to the reduction in skin circulation. The nicotine in the cigarette reduces the thickness of the skin which results to reduction of the elasticity of the skin and causes premature aging.
source:[www.io9.com]