No no. They are talking about habits here. Twins are more likely to have the same kinds of diets and exercises and overall similar lives, which in turn lead to them being in similar shape. It was difficult for them to find divergent twins. But once you are looking at divergent twins they the exercise and lifestyle became quite important.

Your habits and lifestyle are basically pre-determined by a certain age but as the article shows, they are not set in stone because lifestyles may change.