Increased 5-alpha reductase activity with age converts more testosterone to DHT, which inhibits hair growth by reducing IGF-1 and cyclic AMP, leading to pattern hair loss in about 50% of people by age 50.

“Testosterone most people have heard of. Dihydrotestosterone, or DHT, is made from testosterone. There's an enzyme called 5-alpha reductase that converts testosterone into dihydrotestosterone in both men and women. Dihydrotestosterone binds to the androgen receptor at five times the affinity of testosterone, so it is the most powerful androgen in humans, and it is responsible for a number of things that we all really want and like, such as mental vigor, physical vigor, strength, healing capacity, drive, libido, and on and on. DHT itself is not bad. If we take a step back and we acknowledge testosterone levels are higher in males and females at younger ages as opposed to older, but as they get older, there is more 5-alpha reductase activity, which is converting more of that testosterone to dihydrotestosterone and dihydrotestosterone inhibits hair growth by reducing IGF-1 and cyclic AMP, well, then we should all be aboard why it is that by age 50, about 50% of people experience pattern hair loss.”