For me retro depends on the topic we're talking about.
For video games? Well... Games from 15 or more years ago are already retro. Yeah, MGS4 wasn't released in the 90s, but it's been so long since it came out that my life has moved on a lot since then and now Bloodborne is almost 10 years old... But things are different for let's say books, because of how technology advance, for books to be old they need to be at least 100 years old because it needs a change in society for us to see a difference between then and now. We're just starting to see differences with books from the 70s (If they are for medicine then a lot less we need), and with games this happens much faster.