We say, 'You may drink at the age of 21 but not at the age of 20.' Why? Because humans like to create terribly neat categories out of nature because it allows us a nice, tight social organization. The truth is, nature doesn't care that we like nice, neat social organizations. Nature likes variety.
Alice Domurat Dreger is a historian, bioethicist, author, and former professor of clinical medical humanities and bioethics at the Feinberg School of Medicine, Northwestern University in Chicago, Illinois.