The Rock And Roll Years, possibly the UKs biggest contribution to the rise of retro culture) even though it is a term that says nothing of any actual value, unless you use it as a highly weighted code. Actual genre terms can be a highly useful tool to give a rough description of what you can expect to hear and the intent of the content, however the most you can ascribe to a decade is the kind of instruments and recording technology that was available and, broadly speaking, what music hadn't happened until that point. These are not very useful for what the music actually is, especially when you take into account what you are actually trying to say.
A decade in pop culture/pop music is, if people are honest, an incredibly long time. Hundreds of thousands of songs get released in a decade (possibly millions), whole scenes will rise and fall, the zeitgeist will move and mutate beyond all belief. Even if you look a headline acts they don't have half the impact people think they do, Elvis had a career from 53 to 77, but was only culturally relevant for 5 years before becoming a re-run pop-has-been (all be it with some outbreaks of performances and songs) by joining the army, so he doesn't count as being "so 50's" even if you do ignore all the folk music going on around him. The Beatles are constantly considered one of the most "60's" bands ever but were only around for 7 years in the public eye and had a massive contrast in their works. You can't even describe "Sgt Pepper's" as 'the sound of the 60's, as that was only released in 1967. The 70's pretends to be chintz-pop and prog/stadium, but it was also a lot of pub/punk/blues rock and the influence of soul and reggae. The 80's had "New Romantics" for about 3 years, but that covers everything from The Blitz-Kids to Post-Punk to Plastic-Soul so is neither that big a chunk of the decade or that definable a thing, and it totally ignores the nuclear impact of rap, As you go up the timeline towards things just get more cluttered, as recording, promotion, and distribution costs go crashing down and scenes can be created around thousands and eventually hundreds of people from around the globe. Any attempt to encapsulate even a five year period into "a defining theme" is going to fail, hard, as there are just too many sounds and too many intentions going on, so how you are going to double that is anyone's guess.
Having said that it's always possible to go "oh, well I mean the music that was in the charts", but that doesn't work either once you get around to looking at the histories. The bulk of the charts has always been made up of either generic pap that is indistinct enough so as not to cause any offence (and thus noone in their right mind will listen to it again once it's done it's job) or things distinct enough from the rest to be remembered but too much of their own thing to be able to form into a cohesive narrative of what was happening at the time. It's also almost always utterly different to the music that will eventually be seen as important or significant in that time period. For example on sales The Sound of Music is the single most 60's album ever.
What you need to do to use a decade of as a genre is to use it as a code, and what you are using it to do is say "This is how I wanted that period to be, even though I know damn well it wasn't". It's a willful rejection of what actually happened and a replacement of it with your own 'best of' collection (much like all of the retro-scene). It's also an attempt to gain authenticity for your choice, to empower your selection as the most valid version of what happened. It's an attempt to gain simplicity, to reject other songs as not passing the mark so requiring rejection from the records. It's also quite often an attempt to replicate the youth that you miss the most, because everyone is convinced that the music they listened to between 14 and 23 was the most authentic thing ever. And, yes, it's also a quick and simple marketing code to enable folks to go "generic collection of pop you heard from time X", and when have marketing folks ever worried about the terms they use being utterly nonsense?
So is there ever a time to use a decade as a genre? Yes, whenever you are using it to describe a specific style or sound. If you go "Oh, I mostly do 70's ska and 80's electronic" then that actually says something useful and accurate about what you listen to. It shows a distinction, a vintage, and a musics place within the wider context. It also shows that you know what you are talking about, that you care about your music. Because there is nothing wrong with old-school music I listen to them all the time), but there is something about not giving them the respect that they deserve.