For me it's anything prior to 360/PS3, because from then on the graphical and gameplay leaps have been incremental compared to what came before. You really don't consider N64 or PSX retro?
I often thing about this. Technology moved so fast in those days and then it sort of plateaued. Things are certainly getting more powerful of course but nothing like that old generational leap we used to feel.
I think it's also a testament to the age of the medium as well. Most gaming fans are young gen xers or millennials. The medium grew up with its fanbase and with that comes the perspective of a youth. A 7 year old looks back at their preschool years like ancient history, a teen the same of grade school. Combined with how antiquated the tech was it felt holder. Median gamer age is rising and if you ask a 35 year old about when they were 25 it would feel a lot closer.