I think a scent becomes outdated when it was part of the defining trend of an era and that trend passes. You know something is outdated (or perhaps nostalgic) when you know what decade it's from without any prior knowledge. You'll be wrong occationally with this type of stereotyping - but when you are wrong you have found a scent that "smells" outdated rather than "is" outdated. Rive Gauche is a good example - it does NOT smell like a 21st century cologne. I'm not old enough or "well smelled" enough to place it's proper era, but I know it smells like days gone past.
Does that mean you are "out of style" if you wear outdated frags?
Yes and No. There is usually a mainstream style at any point in time, but few people (especially few men) follow too closely to that style. There are lots of elements of men's fashion that tend to be "timeless" or at least evolve veeeery sloooowly. One extra perk is that vintage is in right now and I'm going to make a very dangerous prediction that vintage acceptability is here to stay.