I think it simply means "song that isn't traditional" or "song with a known author"... I don't think the age of the song comes into it. Was it last year when Jim Moray won with a version of XTC's "All You Pretty Girls"? Again not a new song, but a new version that captured the judges' imagination. I'd imagine if someone did a brilliant new interpretation of the songs you mention as examples, there'd be no reason they couldn't be considered for the award. I'm guessing here, though, so could be wrong. I was once before...