FYI: Why Do Old People Get So Hairy? | Popular Science
You’ve seen it at the local pool, at the beach, or even on your own grandpa. Old grizzled men with enough back hair to knit an afghan. Rampant tufts of hair springing out of dark nasal and ear cavities and eyebrows that look Cro-Magnon. What causes hair to grow everywhere but the head as we age?
Scientists don’t exactly know what causes hair to sprout excessively from places like the ears and nostrils but Dr. David Liebovitz, an associate professor of Medicine at Northwestern University, guesses that it has to do with hormones and the lifecycle of hair.
Hairs grow in three stages: anagen, catagen and telogen. First, hair cells grow and divide in the anagen phase. Head hair naturally remains in the anagen phase for an extended period of time, up to several years. Hair on your arms, however, will move on to the catagen phase in a matter of weeks. This is when the hair stops growing and transitions to the dormant telogen phase. The hair stops lengthening and eventually falls out naturally through shedding or external trauma such as pulling.