I don't tend to think of them as retro, even though I realize they've been out for a long time at this point. Relatively, the first two generations became retro much sooner for me. I think the difference was the compatibility with other generations. After gen 2, the game mechanics changed and Game Freak essentially opted for a clean slate; that decision helped make gens 1 and 2 seem obsolete. Gen 3 Pokémon, by contrast, can still be sent forward to X and Y, so they don't seem as cut off to me from the modern games. I think another factor is that thanks to the internal battery saving mechanism in gens 1 and 2, my gen 2 games died years ago (Gold back in '06--it's been gone longer than it actually worked). Dead, non-saving games seem more retro to me. Sure, I've started to get battery error messages in RSE, but it's much easier to ignore the effects of a dead battery in those games, so they seem much more like they did when I got them.
And then, there's the fact that I played the first two generations at the height of the Pokémon craze, or close to it. I associate them with elementary school, and everyone I know playing. Life was just so different back then. Sure, times have changed a lot since I played RSE, too, but in terms of my gaming habits, it just doesn't feel as different. I started frequenting gaming forums and making accounts (not on this site, but others) during third gen. I primarily played by myself, but began to realize just how many people still played from my forum browsing. That dynamic, of playing alone and browsing the internet, has been the same for me from third gen through sixth, but wasn't my experience in gens 1 and 2.
It's not just the age of a game that makes it retro for me; it's the context. Thus, RSE may seem retro to many people in this thread, and I'm sure that there are some people who even think DPP are retro, perhaps a growing number now that wifi is gone. Objectively, perhaps I should see RSE as retro. Sure, the graphics are obsolete, though they still seem vastly different than those in gens 1 and 2. Sure, they weren't designed for a DS family system, and I can't even play them on my 3DS. But in many ways, it still surprises me just how much time has passed since RSE came out. Who knows, maybe I'll see RSE as more retro once the remakes are released.