Can we say unequivocally that green always represents older bile? No, but we can say unequivocally that it is more oxidized bile than the yellow / brown / red bile. Why? Because biliverdin (green pigment) is oxidized bilirubin (red pigment). So the stones that formed with green coloration were more oxidized. Healthier people produce less oxidized bile. Possibly indicative of the overall status of unbalanced oxidation in their body whenever these stones formed, as much as the age of stones (so eat more fruits and vegetables for those antioxidants). It's really speculation that the green bile must be older. In reality it is just more oxidized and indicative of unhealthiness. Most likely, a body with it is dysfunctional and probably retaining more stones.
I felt responsible to answer this since I implied green bile must be older in my 7th flush post, but that was somewhat speculative. If the wikipedia article on biliverdin is correct, it can be derived directly from the old blood cell even before bilirubin, and then be reduced to bilirubin. What we know is green bile must be more oxidized.