By your late thirties or early forties, the extremely thin skin of the eyelids starts to lose its elasticity and stretch. In most people, its the first skin "to go". In the upper eyelid, the crease may be obscured by skin overhang. As the collagen in the dermis starts to be lost, wrinkles and crow's feet appear in those parts of the eyelids that move the most.
The orbicularis muscle thins out and becomes less powerful.
The eyelid support system starts to thin and stretch under the influence of gravity, the stress of blinking, and due to certain environmental factors, most notably smoking and excessive sun exposure. Deeper fat pockets may become visible as they bulge forward.
The frontalis muscle may sag, thus lowering the brow, which in turn pushes the already excess eyelid skin into a more noticeable roll.
The soft tissues of the cheek and midface begin to descend, thereby exposing a small area of relative hollowness below the lower eyelid, accentuating any lower eyelid imperfections, and allowing the edge of the orbital rim of bone to show through the skin.