The American Heritage Dictionary defines era in its second sense as "A period of time characterized by particular circumstances, events, or personages: the Colonial era of U.S. history; the Reagan era. b. A point that marks the beginning of such a period of time." Of course, one means people have naming various eras is by naming them for decades--that is, the Forties, the Fifties, the Sixties, and so on. It is a very convenient means of applying a label to a particular era. Unfortunately, often times a decade does not precisely match the decade for which it was named.
A perfect example of this is the Fifties. When most Americans think of the Fifties, they think of rock 'n' roll, Elvis Presley, poodle skirts, Ford Thunderbirds, drive in restaurants, jukeboxes, and so on; however, the fact is that this popular image of the Fifties in America really didn't develop until after about 1955 or 1956. The early Fifties would probably best be considered a separate era (perhaps the Korean War era) or, at best, a continuation of the late Forties or Post War Era (the era following World War II).
Another example is the Sixties. I would imagine that when the average American thinks of the Sixties, he or she thinks of The Beatles, miniskirts, the British Invasion, hippies, psychedelia, and so on, but none of this was around before 1964. Prior to 1964 the Sixties were just a continuation of, well, the Fifties.
I suppose that it is just a fact of life that various eras do not always conveniently coincide with human time keeping. Wars do not always begin at the start of a decade and end five years later. Fashions come and go as they please. Musical styles change regardless of whether one decade turns over into another. Irregardless, most people will probably name eras for the decade in which they take place...