I've always felt that, from a culture standpoint, the decades run from midpoint to midpoint. What we think of as the 60s music and style really stated in 55 and went to 65. What we think of as the 70s really went from 65 to 75, etc...
Of course, this isn't entirely accurate, either. The truth of the matter is that taste, culture, style, fashion, etc... aren't rigidly bound by the year but grow organically over time. 1969 wasn't much different from 1971. It's not like everyone goes "It's 1970! Quick, let's remodel completely!"
I completely agree.... the early 80s fit more with the late 70s, and the late 80s melds into the 90s pretty seemlessly. I've always considered "my" era to have been 85-95 (age 7-17) as growing up... neither an 80s kid or a 90s kid. Half the things that are considered "80s" I did not know nor care about at such a formative age lol.