The Cardinal Electors are those members of the College of Cardinals who have not reached their 80th Birthday on the day a "Vacancy of the Apostolic See" occurs, whether by the death or the resignation of the Supreme Pontiff. That is, if they turn 80 years of age on or before the day a Pope dies or resigns, they are ineligible to vote in the Conclave to elect his successor. However, if they turn 80 years of age the day after the Pope dies or resigns, they are eligible to enter the Conclave and vote. Interestingly, even if a Cardinal cannot vote he can be elected.
Cardinal Electors of the Holy Roman Church
As of Sunday, December 04, 2016, there are 220 Cardinals,
109 of whom are eligible to vote
and 111 of
whom are ineligible.
In the Conclave of April 2005, 115 Cardinals voted.
Joseph Cardinal Ratzinger was elected Supreme
Pontiff on the 4th ballot.
In the Conclave of March 2013, 115 Cardinals voted.
Jorge Mario Cardinal Bergoglio was elected Supreme Pontiff on the 5th ballot.