Isn't this a constant sliding scale?

When I was young, Atari 2600 was "retro", SNES/Genesis was recently replaced and PS1/Saturn/N64 were "modern"
When I was a teenager, stuff like NES was just becoming "retro" and PS2/GCN/XB were the new "modern"
Now consoles like the PS2/GCN/XB are starting to become the new retro while PS4/NSW/XBO take over as the new modern consoles.

These modern consoles will also become retro in time, so its really strange to make some fixed point in video game history the barrier between the two labels.

Same. I'd classify as ancient era (first Atari to SNES), old era (N64/PS1/Saturn and Dreamcast/PS2/GC/XB) and modern era (PS3/X360/Wii onwards). When we jump to native 4k next gen I'd call it contemporary era.

In my life I have expereinced 4 diferent eras.

Era 1 - Atari, extreamly simple 2d flat plains, massively simple controls.
Era 2 - NES - PS1, Advanced 2D/ etreamly basic 3D or simulated 3D depth, more advanced controls.
Era 3 - PS1 - PS4, Fully realized 3D worlds, Highly refined digital controls.
Era 4 - PS4 - ?, Virtual Reality, Combanation of Physical controls, and refined digital conrtols.

Modern gaming started in 2016 with PSVR, Oculus Rift, and HTC Vive. Everything else is retro to me. I still get excited about stuff like God of War, The Last of Us Part II, Mario Odyessy, and such, just like I do when I see Goldberg, Barry Bonds, or a classic movie, but it is just a nice return to nostalgia. The wonder and amazement in gaming now comes with VR stuff, and where games can go in the future with this incredible new tech. As I have said many times the last 3+ years, VR has me feeling like a kid again, dreaming up all the possabilities about where games and tech can go. VR is modern and futuristic at the same time, and I absolutely love it.

As a collector, retro is an ever increasing window for me. Wii and prior is retro to me. PS2 and prior is retro. XBOX is retro.

I pretty much see the start of the modern era at the dawn of the 2000s, but maybe that's just cause I'm old..

Pretty much MS stepping in sealed it. I just don't see Xbox as retro, as its games have a lot more modern characteristics, as does PS2 to A lesser degree. I kind of see Dreamcast as the final "retro" console, which is fitting because it was Sega's last. Original PS is odd in that it feels sort of like a retro console with some modern characteristics. It's sort of "proto" modern, but many of its games are still very retro despite its emphasis on 3D and CG

Modern ear would be back during n64/ps1 days cause that is when 3d really started becoming a thing.

I suppose for me, it would be 64/PS1 and before. GC/PS2/XB doesn't seem quite that old yet.

Since I only do handheld gaming, I consider the Gameboy Advance as the end of retro gaming since the jump from Pokémon 2nd gen to 3rd gen was probably the biggest of any kind in the Pokémon mainline franchise.

I grew up with the Gamecube so I always considered anything 6th generation and above to modern. Everything before that was retro.

When I was younger I considered the N64 as "That generation that was before mine.". I also considered the NES/SNES generation as "An ancient time period where 3D was not invented". The Age of Atari and before I considered as "The Land Before Time and when Dinosaurs ruled the land and where writing was not invented. ". (I didn't know who an Atari was but I knew who Pac-Man was!)

As a guy who was born to catch the TAIL END of the gamecube, anything behind the wii is retro. While I consider NES and SNES classic.
Hell I was a pretty dedicated gamer but I didn't even KKNOW about sega and sonic untill the olympic games came out lol.