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I don’t remember exactly, but it’s not testosterone itself that causes the hair loss, but what your body converts the testosterone into.

And that process can ramp up as you get older depending on your genetics.

So more testosterone, more byproducts, more hair loss.

Testosterone gets converted into DHT, and DHT causes male pattern baldness. Maybe younger males don't convert as much testosterone into DHT as older guys do

DHT also stimulates beard growth, so at least you get a return on investment.

For one, it's also about the hair follicle's sensitivity to circulating androgens. Maybe that changes with age.