Why do people say AMD cards ages better than Nvidia? I would assume the card that has better performance would age better (?)
Its a curious notion for sure.
Back when AMD were overbuilding their hardware, it took months and sometimes years for the drivers to catch up, and so the fanboys fell back on the idea that AMD performance improved over time like fine wine, instead of acknowledging that AMD couldn't get their drivers optimized in a timely fashion for AAA games.
The fine wine argument is a reaction to AMD having slow driver releases, because if AMD didn't have slow driver releases then there wouldn't be any "fine wine" improvements later. You can't have both, they're mutually exclusive.
The only other aspect is the amount of VRAM that AMD uses compared to Nvidia. Go all the way back to GCN 1.1 with the R9 290 and there were 4GB and 8GB variants, while Nvidia was playing around with 3GB 780s and 6GB Titans. As far back as I can remember, AMD have always had more VRAM. I think the VRAM size might be the only meaningful way that AMD cards could age better, but at some point all the VRAM in the world isn't going to give you more performance, and in my own testing Nvidia manages VRAM usage better than AMD does, which means AMD having more VRAM might simply be to compensate for less aggressive memory management.