Here's something from A Suitable Wardrobe last year that will answer your questions.
Mohair has been used in the West since the 16th century, when
Charles V first brought angora goats to Europe. The delicacy of
the species made mohair relatively rare as well as expensive and
five hundred years later it is still a luxury fiber, like cashmere and
silk, and considerably more expensive than most wool.
It is more expensive because, young or old, those delicate...