When your 90+ years old and jamming a N64 game into the M.5 slot of a PS7, out here asking your grandson shit like "why the fuck isn't Metal Gear Solid 6 : Daughters Of The Deep State not working jr"
only to realize its Goldeneye 64 and you are screaming into a Wendy's Bank ATM (in the future, Wendy's becomes the biggest bank buying out Bank Of America, Chase, all cause they went back to the classic yellow cause it hit different)
jk aside, imho a game becomes too old for you when the design is too dated to be enjoyable, now this is subjective to many I understand. When we are done with this gen and are used to damn near 1 second loading, it will be hard to go back to waiting minutes or something, as we go deeper into generations, that will be much hard to return to for many.
The next point I'd say is work arounds. How many work arounds is needed to have the game run? Some PC titles I have a note pad where its like "turn off Vsync, only play on medium to avoid white screen, runs better on Windows 7, start with save file 12" or some shit. Depends on how many steps, if a remaster or "enhanced" version exist that is more modern etc.
Next I'd say is the display output. Does it NEED some CTR tv to play based on compatibility? They do have converters for this as I own one to use for Gamecube, N64, Dreamcast etc
For the most part, I really feel it comes down to design. We are used to fast loading (or going to be) cameras that make sense as we don't even fucking debate or rate that show now days that so perfect and set in stone now, before that was factored a lot, now every game has a 3D camera angle that makes sense by default and I can't tell you 1 game in the last 15 years where that was some massive issue or something. So I think action games get the worst end of this as at the time they were cutting edge, to todays standards, they are just awful to play.
turned based RPGs imho age maybe the best in this regard as most of the game is conceptual, as oppose to some exact action thing that will be dated, so the worst part about any of those older games really is the loading, if someone can suffer thru that, they should still be able to have a pretty good time, everything else? Maybe not.