It is commonly thought, and asserted in the Meejah, that a Bishop is obliged to submit his resignation when he has completed his seventy fifth year of age.
This is not quite accurate.
What Canon 401 (1) requires is that the Bishop rogatur [is asked] ut renuntiationem ab officio exhibeat Summo Pontifici. This is based upon Christus Dominus  of Vatican II, which is vaguer; without mentioning any particular age, it merely says that when the bishop has become less capable of fulfilling his duties properly because of age or some other serious reason, he is asked to offer his resignation.
Christus Dominus does put an adverb before rogatur: enixe [earmestly].
Canon 401divides the requirement into two halves: (1) the Bishop who has reached his 75th birthday is asked to submit his resignation; (2) the bishop who is ill and weak or has another gravem causam is earnestly asked to submit his resignation.
You are probably wondering ... no; the 1917 Code has no comparable provisions.
Canon 401 makes the request stronger if the bishop is old, ill, or weak, than it is if he has merely reached a specific age. rogatur, without the enixe, is not a particularly powerful word. English translations of the Conciliar documents and of the Code of Canon Law usually translate it as requested.
Before the Council, most bishops, like the popes, died of old age in possesssion of their sees (having been given a coadjutor if they had become particularly frail). This would respond to the old conviction that a bishop was wedded to his Church and would be an adulterer if he were translated to another See. The twentieth century provisions forget all this and treat the post as if it were a matter of efficiency and competence, like being a supermarket manager. Having a spot of trouble doing the job? Out you go! Good sales returns? Excellent! Have a bigger shop or become regional manager!
In my view, this is untraditional and, I would go so far as to say, corrupt.
To be continued.