The perfect answer to this question is demonstrated by a case update I received from the Employment Lawyers Association, referenced below. The same employer was found to have objectively justified its mandatory retirement age in a case brought by one its employees, but not in a case brought by another of its employees. 

What was found to be proportionate in one case, was found to be disproportionate in another, on the evidence presented to the tribunal. So often our job is to look behind the questions, resisting straight yes or no answers, and to investigate the facts and circumstances, to advise you on risk and likely outcomes. Challenging and interesting, but not easy!