Well, retro doesn't mean a style of games. I believe its more of an age thing than a style thing. The biggest mistake we make when classifying is "Are they retro, or are they not?", where the better question would be "Are they retro, or are they not? If they are, then by what degree?". Even if two games are considered retro, the generation they belonged to and how it differed from other gens simply do not fade away to become "one with the retroness".
Technically speaking, anything in the past counts as retrospective. Suppose a new Halo game comes out, take Halo 4. Anything they do to compare Halo 4 with the past titles is holding the previous titles in retrospective. Even the most recent one besides Halo 4. But when it comes to pop culture it's usually "The decade before the last and backwards", which could be the same for gaming--"The gen behind the last and backwards".