How old are you? The more thought you bring to bear on the question, the harder it is to answer. For we age simultaneously in different ways: biologically, psychologically, socially. And we age within the larger framework of a culture, in the midst of a history that predates us and will outlast us. Looked at through that lens, many aspects of late modernity would suggest that we are older than ever, but Robert Pogue Harrison argues that we are also...