The proper term for the lack of a sense of smell is Anosmia, and as of 2000 it affected less than 1% of Americans.
Most people with anosmia develop it following a head injury or illness, with fewer developing it due to exposure to toxins, and only 1% due through aging. This often leads to severe depression and similar mental illnesses.
Some people may develop anosmia due to nasal polyps. This cause is relatively easy to treat.
Around 2% of anosmia 'sufferers' are born with the affliction. Unlike people lacking other senses, Anosmiacs often don't realize they lack the sense until around or past puberty. Many state that they felt they would 'learn how to smell' as time went on.
NO one is quite sure as to the cause of Congenital Anosmia, but it is believed to be the result of a hereditary gene malfunction. This is supported by claims by some Anosmiacs of learning of more Anosmiacs in their families once they themselves admitted to having the affliction.
Anosmiacs sometimes use the term 'Olfies' to refer to people with a sense of smell, while the term 'Headbanger' is used to refer to other Anosmiacs that lost their sense of smell due to an accident.
Several spell-checks don't register Anosmia as a word.