I don't think it's a matter of older films not being worthy of appreciation, or 'aging' well. I just think the best films aren't always the ones that make the "Greatest" lists, and therefore naturally can't live up to prolonged scrutiny.
TIME Magazine had an interesting article recently highlighting past instances where 'superior' films and actors were passed up for Academy Awards. Interestingly, many of the same names listed above are also mentioned by TIME:
http://www.time.com/time/magazine/article/0,9171,1713487,00.html
Another factor, with so many films to choose from, so many different "Top" lists, and peoples' different expectations of what a 'great' film is, is it any wonder that each of us can't agree with every single one of them? Here's a typical "Top 100 Films" list:
http://www.time.com/time/2005/100movies/the_complete_list.html
Some real treasures listed here that people will still be watching 100 years from now. Some films that I think aren't so great. Some blatant errors and omissions. And even a few that I've never seen.
Genres and subject matter make some of the old "great films" seem pale after time. War films, sci-fi movies, and social-issue films especially have a hard time keeping up. The advances in television drive other films into the ground - is
The French Connection any more 'gritty' than an episode of
NYPD Blue or
The Shield, for example?
The exact opposite is true for other themes - even the best modern detective/mystery movies are blah and soul-less compared to original film noir stories, even those considered mediocre at the time.
Which film HAS aged well in your views?
Bogart's great efforts:
To Have and Have Not,
Maltese Falcon,
Key Largo
Hitchcock's classics, especially his older B&W releases.
Some comedies - I have no doubt that people will be laughing at The Three Stooges for generations to come.
Film Noir. When someone tries to 'modernize' it, they end up creating a sad imitation almost every time. Not even
Chinatown can compare to the real thing, like
DOA,
Scarlet Street, or
Detour.
Perhaps a lone western, or two?
High Noon, for example.