For the first time, the middle-aged make up the biggest, richest and most influential segment of the United States population.New York Times reporter Patricia Cohen explores the concept of middle age, and our perceptions of it, in her book called In Our Prime: The Invention of Middle Age.Cohen will discuss her book in an appearance Wednesday, July 18, at the Louisville Free Public Library (the event is free, but tickets are required; call 574-1644).She speaks here with WFPL’s Rick Howlett.