Copyright laws originated in England because Charles II was worried about all the book copying going on, and established a license regulation dealio, which eventually matured into actual copyright law. Later there was a conference in the 1800s which dealt with international copyright issues. (interestingly, the US and the UK didn't sign on until the 1980s thereabouts) But as far as "oldest" copyrights only last for either 50 or 70 years after the author's death, although it varies by country. I'm pretty sure that in the US it's 50 years after death for most things.
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so... the answer to your question then, would be: every song that's 50 years old! At current, however, in the US, the ties for oldest would be songs that were published in 1923, where the author has not been dead for over 50 years. :) enjoy!!

it's author's life plus 70 years in the US. but the copyright on works for hire (like articles in a newspaper, or songs written for an artist by a songwriter) expire 95 years after publication, or 120 years after creation, whichever is shorter.
FURTHERMORE, works can be renewed, as in by the estate of the author. the lobrary of congress' site is hard to understand here, but it appears that the longest you can copyright something is 95 years total.
so i would assume that the oldest copyrighted song is around 95 years old.

except that like i said - nothing before 1923 is copyrighted in the US, which means that even if you're right, that's still only 84 years. Also, I just did some more checking, and it looks like the 50 year limit was a 1976 law, and in 1978 they extended it to 70 years after life of author.. 120 years is for corporate authorship, and it's only after creation, whereas its only 95 years after publication (again, it's also whichever is shorter)
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...btw, the name of the '78 copyrights law is actually the Sonny Bono Copyright Term Extension Act - how many people get to have a copyrights law named after them? super exciting neh?

Thanks all. I was pretty sure about the author's life bit, and also the ability to extend a copyright. So I suppose a 1923 song by a particularly long-living person (or stubborn estate) could be clinging to some songs.

sung to the toon of oh susana..

Grab yer ankles
you live in USA.
Obama is in the house
and now you gots ta pay
Bush and hiliary….

you get the idea.