I would probably say 2002ish
Like, anything older than '92 or so.
Though animation and video quality do play a part in it. Gundam War in Pocket certainly feels newer than both Victory and G. The original Ranma 1/2 releases (essentially any clip you can find on YouTube) make the anime feel old (thus adding to its anemoiac quality) while the crispness of the Blu-Ray release makes it feel much more modern.
Certain 2000s anime are a weird case. While I don't think of them as "old" perse, they certainly feel off and I can immediately tell that they're 2000s anime.
Mmm... I think pre-2000s counts as old for anime.
Then, stuff from the 00's that's in a square ratio is "kinda old" and stuff from then that isn't in a square ratio is "sorta maybe old."
I get using aspect ratio isn't necessarily a useful quantifier for age of an anime, since some studios/teams shifted earlier and later and stuff, I think, but I think the ratio does at least affect how old it feels. And that sort of feeling is important for an open-ended, opinion-based question like this.