Age. “As we get older, the collagen fibers that support the skin and give it a uniform texture start to unravel,” says Kenneth R. Beer, a clinical instructor of dermatology at the University of Miami, in Florida. Broken-down collagen causes the surface to become rough and uneven, so it doesn’t reflect light as well as smoother, younger skin.

Free radicals. “When your skin is assaulted by free radicals (damaging molecules in the air that are formed by smoke, UV rays, and pollution), its cells oxidize and become compromised,” says Beer. That translates to skin that is rough, dry, dull, and less able to reflect light effectively.

Skin shade. “Light-colored skin tends to lose luminosity more quickly than darker skin tones,” says Beer. Skin that is on the fair side has less natural UV protection than skin that is more pigmented, and years of sun exposure can cause pale skin to become thinner and less luminous.