1988 - Welcome to the wild world of computer animation
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Do you base it solely on age? On the year/console of release? Graphical style? Vibes? Divination?
Additionally: based on your criteria, what are some complicated edge cases?
For example: is DOOM 3 "retro" because it's over 20 years old and its series was rebooted, or is it modern because it's got nice 3D graphics and lighting and whatnot? Is Crow Country retro even though it came out last year?
The point of this isn't to find the hard line of what is/isn't retro -- it's to play around in the gray areas for what "retro" potentially does or doesn't describe.