It's the same thing with decades. People often say the "sixties" didn't really start till 1963. And when you think of the start of 1980's culture, a lot of people are really only talking about 1983-1984.

Like, 1960 itself clearly belonged to the 1950's, the same way 1980 still belonged to the 1970's -- culturally, that is.

Obviously, the question of what year a decade "really" started in, allows for endless argument. :)