Ah, sorry about that. Here's an overview of the decade rule across the field: http://english.stackexchange.com/questions/13631/is-an-apostrophe-with-a-decade-e-g-1920-s-generally-considered-incorrect/299512#299512
AP & Chicago say no apostrophe,
But Words into Type, third edition (1974) takes the opposing view:
In referring to decades, the sixties or the 1960's is generally preferred (not '60's, '60s, 60's, or 60s; the last form is used occasionally for ages of persons).
The New York Times Manual of Style and Usage (1999) agrees with Word into Type about the apostrophe, although about little else:
decades should usually be given in numerals: the 1990's; the mid-1970's; the 90's. But when a decade begins a sentence it must be spelled out. [example omitted]; often that is reason enough to recast the sentence.
It's probably true that no one cares what NYT Style guide says about anything, and most of the guides that matter say no apostrophe in decades. But it's still not wrong to use an apostrophe to pluralize a decade.
I normally adhere to Chicago style, but over the last couple years I've become personally uncomfortable with both representations of plural decades and try to write around them entirely.
Oh, I thought of another sticky situation--surnames. You invite your coworker Jimmy Othello & family over for a BBQ. Who's coming?
The Othellos
The Othelloes
The Othello's
None of those is incorrect. In most writing, #3 would be the most clear to the reader, imho. It looks goofy, but they all look goofy.
Othello isn't a common surname. Without the apostrophe the reader can't know whether it's the Othello family, or the Othelloe family, or the Othelloes family. It is OK to use apostrophe plurals to remove ambiguity.