Genes and hormones alone are not enough to cause hair loss. Time is also needed, the amount of which is necessary for hair loss to begin varies from person to person.
Susceptible hair follicles must be continuously exposed to DHT over time for balding to occur. The age at which all the variables, such as enough testosterone in the bloodstream, the presence of the enzyme 5-alpha reductase in the hair follicles, and the follicular sensitivity to DHT are all determined by our genes.
The rate of loss also varies from person to person, and is characteristically irregular, with men losing hair in alternating periods of slow and rapid loss, with periods of stability. Why the rate of hair loss varies and alternates is not yet known. We do know that that over time, as a man ages, the volume of his hair will decrease, irrespective of genetic predisposition. It’s the amount and rate that varies and depends more on genetics.
A man’s entire scalp will experience a decrease in the amount of hair volume, even the “permanent zone.” That said, enough hair usually remains in the “permanent zone” so that hair transplantation remains a viable option for men well into their 70s.