And in the man, the voice breaks, the beard begins to grow round the lips and on to the throat.

There are the obvious physiological changes resulting from the gradual bursting into free activity of the hypogastric plexus and the sacral ganglion, in the lower body, and of the cervical plexuses and ganglia of the neck, in the upper body.
Why the growth of hair should start at the lower and upper sympathetic regions we cannot say.

Perhaps for protection.

Perhaps to preserve these powerful yet supersensitive nodes from the inclemency of changes in temperature, which might cause a derangement.

Perhaps for the sake of protective warning, as hair warns when it is touched.