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][1]". And while the most favoured answer back then focuses on the fact of real, every day usage of otherwise outdated systems, todays concensus incooperates a wider range of outdated systems, but especially theoretical and research questions about old software, hardware and culture in general.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)

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*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_"

  [1]: https://retrocomputing.meta.stackexchange.com/questions/2/what-constitutes-retro/11#11