I typically don’t use an apostrophe with plurals in any situation, but I always assumed that the use of an apostrophe in constructions like acronyms:
Forty BA’s were given out to students this year.
or numbers:
Though the greatest period instability occurred during the 1950’s when. . . .
was more or less ok. I would personally use BAs and 1950s here, but I've never thought the apostrophe in this case was necessarily “incorrect”. What say ye?
Best Answer
My answer focuses on the header question about decades—which is the question that most readers will probably expect to find answers to here. With regard to decades expressed in numerals rather than spelled out in letters, some style guides recommend omitting an apostrophe, while others recommend including it. For example, from The Chicago Manual of Style, fifteenth edition (2003):
The Associated Press Stylebook (2002) adopts a rule very similar to Chicago's:
The Oxford Guide to Style (2002), while differing with Chicago and AP on capitalizing the spelled-out decade name (Oxford University Press prefers this) and on using abbreviations like '60s (OUP condemns this), agrees with Chicago that the plural numeral form should be spelled without an apostrophe before the s:
But Words into Type, third edition (1974) takes the opposing view:
The New York Times Manual of Style and Usage (1999) agrees with Words into Type about the apostrophe, although about little else:
Clearly the question of how to render a particular decade in print is a style issue on which reasonable style guides may differ. Follow the one you have to follow, or choose the one you like.