Aging of the human skin is a natural phenomenon that usually results from constant exposure to intrinsic (genetic factors, time, hormones) and extrinsic factors (UV exposure, tobacco, pollution). In areas or parts which are frequently being exposed to the sun, photoaging combines with the process of intrinsic aging, thus resulting in an increased number of senescent cells and subsequently accelerates the aging process. 

The severity of photodamage is mostly dependent on constitutional factors, such as skin phototype (tanning capacity, skin color), intensity, and duration of sunlight or UV exposure. Aging generally affects almost every aspect of cutaneous biology, which includes pigmentation also. Clinically, the phenotype of pigmented skin of elderly has a spotted, uneven color, primarily due to age spots, with or without hypopigmentation. Uneven pigmentation could be due to the hyperactivation of melanocytes, altered pigment distribution and turnover. 

Furthermore, regarding direct damage to pigment-producing cells, photodamage also alters the physiological crosstalk between keratinocytes, endothelial cells, fibroblasts and melanocytes that are responsible for natural pigmentation homeostasis. Age-independent longwinded appearance of senescence-associated markers present in the dermal and epidermal compartment is associated with vitiligo, which suggests that premature senescence might play an important role in the pathology.

Source: Kang HY, Lee JW, Papaccio F, Bellei B, Picardo M. Alterations of the pigmentation system in the aging process. Pigment Cell Melanoma Res. 2021 May 28. doi: 10.1111/pcmr.12994. Epub ahead of print. PMID: 34048137.