A world where, instead of aging in years, the people age in knowledge. So, instead of dying once a general age was reached, they die once they achieve a certain amount of knowledge. A man could live forever if he never learned anything, or could die after reading a few choice books. Of course, the knowledge of the land would be regulated, like medical drugs.
"The Cetemi have a most curious custom that in their celebrations must all men don the garb of women and women... the garb of men, to the aim that none shall... know another." -An Account of Barbarian Lands, Lord Shakthur