Collagen Loss is a Key Factor in Aging

As skin ages, the dermis thins due to collagen loss, thus reducing its ability to retain elasticity (from elastin) and moisture (from hyaluronic acid). In addition, facial fat moves and resettles, changing the facial structure. While these changes are occurring in the dermis and subcutaneous layers of your skin as it ages, the epidermis changes color and texture. Over time, bone is reabsorbed, becoming thinner and more brittle. This causes your face to appear thinner and results in a weaker-looking jaw. Osteoporosis can compound these effects, as the bones thin faster.
Due to a combination of factors, skin texture and color also change. The skin becomes rougher and drier, and blemishes and discolorations appear. They appear first as light blemishes and changes in pigmentation. Later, raised spots of pigmentation may appear along with continued discoloration. Skin also increases in redness. Veins become lightly visible, becoming more visible with advanced aging. Collagen loss leads to loss of elasticity and moisture
The loss of collagen results in the loss of elastin and hyaluronic acid, as they need collagen to provide the structure for their retention. This reduces the moisture, suppleness, and elasticity of the skin, and causes thinning in the face, called volume loss.
The face also thins due to fat loss, as well as the movement of fat to other areas of the face. Skin appears looser and sags. The diminished elasticity of the skin reduces its ability to retain its shape and it does not conform as closely to the contours of the face. Gravity then pulls on both the skin and the underlying fat, and they move down your face. This results in sagging eyelids, bags under the eyes, and jowls.
There are two types of factors in facial aging, internal (or chronological) and external (or environmental):
Internal factors are those that happen to you over time, primarily caused by genetics and natural aging processes.
External factors are things in the world around you that have an impact on aging.
These two factors result in non-visible and visible effects on your face. Visible effects are those you can see with the naked eye. The speed at which these changes occur varies depending on the scope and extent of environmental factors and genetics.

Visible effects of aging are those that you see when you look in the mirror:
Wrinkles, lines and folds
Loss of elasticity and sagging skin
Skin texture and color changes
The visible effects follow a progression as you age, dependent upon genetics and external factors

Advancements in science and development of new treatments provide a greater breadth of options to counter the visible signs of aging. Non-invasive treatments are now available that can be administered in a visit to the doctor and result in no down time afterwards. The effectiveness of each treatment will vary and it is important to discuss your expectations with your doctor. To keep informed of new treatments.
Facial rejuvenation treatments can be classified in three categories:

Resurfacing, which changes the surface of the skin;
Injectables, which can fill lines, smooth wrinkles, and replace collagen, as well as "relax" overactive muscles; and
Surgery, which can lift sagging skin, remove excess fat, and fill hollows. (source:agingwithbeauty.com)