Everyone will have their own opinion, so the best answer is- "it depends."
I disagree that it's based solely on the age of the device or game. I learned that the hard way when I backed "Retro" gaming mag on Kickstarter, believing that they were going to provide content on classic games. Then when the magazine came in it was mostly about new games being done in the retro style. So for some people "retro" is just a visual or gameplay style. Minecraft is "retro."
What do I consider to be retro? My definition: 1) any game that doesn't use polygons; or 2) if it does use polygons, anything that's 2D or 2.5D
I don't consider something like, say, Goldeneye on N64 to be retro. It's definitely old, but I would argue it's a modern game in every sense of the word. Something like 3D Dot Game Heroes is retro even though it's newer than Goldeneye because it's 2.5D despite what its name suggests.