As a person ages, hormonal changes occur which contribute to 3 major aging effects on hair: thinning, graying, and dryness of hair/scalp.
Hair thinning from Aging
The diameter of the hair fiber is genetically based and controlled by hormones and receptors. In the hair thinning process, there is a decrease in the number of follicles and a decrease in the follicle size. Typically, the problem arises because the anagen (growth) stage gets shorter. If the anagen stage is short, the hair fiber cannot reach its maximum diameter before it sheds again (telogen phase). Furthermore, it doesn’t reach the maximum length before it sheds. This is why with age, it is harder to grow hair past a certain length. Note that this maximum length varies from person to person.
Hair graying from Aging
Gray hair may relate to the size and distribution of the melanin granules as well as the types of pigments in the fibers. However, gray hair is more dependent on the production of less pigment in the individual hairs with advancing age. Graying can occur at any age, but usually occurs more around middle-age onwards. [More explained here].
In one study , researchers suggest that gray hairs are more coarse and more wavy than heavily pigmented hairs on the same person. In fact they believe that this increase in fiber curvature (less aligned fibers) could be the reason why gray hair often lacks lustre. The nonpigmented hairs are also more sensitive to light radiation and will get damaged more easily. Higher levels of cysteic acid from damage were found in gray hair fibers exposed to ultraviolet radiation for a long time.
Hair dryness from Aging
Sebaceous glands occur over most of the body where hair fibers exist.These glands excrete their oils through the narrow opening of the hair follicle onto hairs and the skin surface. The output of these glands is related to the size of the gland, age, and sex.
With increasing age, (40 years and above) the sebum output decreases, more so in females than in males. Some sebaceous compositional changes have also been associated with age. Oily hair is related to the composition of hair lipids as well as to the amount of lipid on the hair. With less lipids present, hair and scalp feel drier. Additionally, this means that the hair receives less protection from external damaging factors such as heat styling or excessive combing. It also means that the damaged hair can thus lose moisture more easily, compounding the effects of dryness.