I don't really think that the age of a song has to do with its greatness. Their are plenty of older songs that I think suck, the same goes for newer stuff.
A lot of the time it's the elder tracks that are better because you know some have been listened to for years and are still considered as classics - (The Clash, Bob Marley) etc.
Some of them really remain since they were very good. Others are just forgotten. I have still chosen the immortal option since there are some oldies that will never die
The songs that have lasted this long from past decades are genrally the ones that'll stay. The fact that are still played is testament to that. No matter how "unfashionable" they seem, there will always be someone listening to it, until it comes around again.
i find that some not ALL old songs are immortal. they hold in themselves a part of the past-the time when it was written and sung and the emotions that are there between the lines.