Why do men lose their head hair as they age but not their beard or pubic hair?

Because dht doesn't gather there. Dihydro testosterone.

Also, we do lose some hair density as we age. The beard usually doesn't, but the pubes often do. But that's more a matter of it growing back in slower as the body slows down. I'd say it's an 8/10 thing where's there's visible decrease in pube density.

Used the be a nurse's assistant, so I've seen plenty of naked people lol.

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IIRC... natural testosterone production leads to a biproduct called DHT. It's a chemical responsible for many adult male characteristics. Beard hair is different to scalp hair. There is evidence this chemical causes scalp hair loss, but beard and body hair growth