Skin aging is a biological phenomenon due to age.
External factors - like free radicals - and skin structural modification can speed the process up, making collagen and elastic fibres thicker (from 20 years old on). Collagen fibres metabolism of synthesis and degradation slows down while soluble collagen increases. Fundamental substances seem to reduce: glycosaminoglycan and Hyaluronic acid reduce and polysaccharides metabolism slows down.
As skin ages, sebaceous and sudoriferous secretion lower, bringing to dehydration. Melanin production lowers, predisposing the making of dischromia, so healing process slows down and complexion becomes greyish.