Sorry if this has already been answered. I haven't really figured out stackexchange yet. But what time period is retro computing? From the beginning to the 80s? 90s? Early 2000s?
This is a question that comes up every now and then - in fact, one of the very early ones on Meta.RC.SE already asked "What Constitutes Retro". And while the most favoured answer back then focuses on the fact of real, every day usage of otherwise outdated systems, todays concensus incooperates a wider range of outdated systems, but especially theoretical and research questions about old software, hardware and culture in general.
While age does not scale as well as a first thought may imply, there is a general notion that systems less than 20 years old do need a good reasoning to be on-topic. Also, questions involving main stream architecure like a 1999 PC will have a harder time than obscure offside developments like a Motorola Marco.
The defining point is also less the exact date of a certain hard or software, but rather the intention and focus of a question. Emulators are eventually the best example here: Questions that would have been the same, with the original hardware (*1) are without any doubt on topic. Further some about the emulation could be on-topic (*2,3). But questions about handling of an emulator and interaction with modern hardware (*4) are complete off-topic.
Bottom line: RC focuses more on content than any arbitary time stamp.
(For a wider perspective, try a search for "topic" on Meta.RC.SE)
*1 - "How to save a source in BigMac to be read later on by AppleWriter"
*2 - Like asking how a certain instruction should be emulated.
*3 - I wouldn't even mid questions about emulation techniques, as they, even while targeted and tied to modern software development, are relatable.
*4 - "How to operate my USB3.0 hyper reaction time joystick with MAME on Windows"