: relating to, reviving, or being the styles and especially the fashions of the past : fashionably nostalgic or old-fashioned <a retro look>
— retro noun
Off the dictionary.
So I would put the distinction on any sort of significant technological innovation. What's retro is prior to any innovation, and what's not retro after said innovation. For example, retro cartoons were before there were color. We could consider retro games reminiscent of before there was 3d, or each platform could have their own specification of "retro" if they were around long enough, e.g PC.
>>61 could have a point.