From a reader…
Can the Pope “change” or “raise” the required age of Cardinals
entering a Conclave, say to 85 years old, or even 90? He seems to be changing other laws all by himself, why not this? Does he have the power or authority to do this?
Yes, a Pope can change the “cut off” age for Cardinals to be able to enter and to vote in a Conclave. In 1970 Paul VI established that only those Cardinals who had not yet reached their 80th birthday at the time of the end of a pontificate could vote.
For example, say that Pope Clement XV dies on 1 April. That begins the interregnum period of at least 15 days that resolves into the Conclave to elect a new Pope. Benedict XVI established that if all the Cardinal electors were in Rome sooner, they could start sooner. But while they have to wait at least 15 days, by 20 days they have to start the Conclave even if some electors are not there yet. So, after Clement XV dies on 1 April, on 2 April Uriah Card. Heep turns 80. Card. Heep can enter the Conclave and vote, even though he is 80 when the Conclave begins because he turned 80 after the death of the Pope. Had Heep turned 80 on 31 March, no dice.
Historically, there have been really long interregnum periods, “sede vacante”. Back in the 13th century there was one break, between Clement IV and Gregory X that lasted almost 3 years.
Back to the question. Yes, a Pope can change the age of voting Cardinals, electors.