Why do hairs on the back and sides remain?

[NotBelievingIt]

While I don't subscribe to the galea theory as the only thing for baldness, its obvious it isn't, but overlooking the perfect match up of the galea with hair loss suggests that there must be a correlation.

I would be most curious if DHT sensitivity is developed over time, or is it basically there from birth? If its actually developed as time goes on, what is triggering it in the first place?