> At what point do Beatles songs go from being old to new?

Right around when they start using instruments of a nonpercussive nature, I imagine. Their early songs are all rhythm, but their later songs have some piano and else in them.

Another, perhaps less subjective, distinction would be that early Beatles is pre-"Revolver," and late Beatles post-"Revolver." Or, at least, that's where I'd put the line.