"Since the pastoral office of bishops is so important and weighty, diocesan bishops and others regarded in law as their equals, who have become less capable of fulfilling their duties properly because of the increasing burden of age or some other serious reason, are earnestly requested to offer their resignation from office either at their own initiative or upon the invitation of the competent authority"

This article led to the requirement that bishops and priests offer their resignation at 75

But one big fish was missing: the pope.  He can continue in office until he dies....with the resulting problem that his last years in office see other people make decisions in his place - people like (Abp) Georg Gaenswein

See: Why Pope Benedict should retire soon

Jesus, thank you for encouraging Pope Benedict to resign gracefully.