Cardinal Electors

The Cardinal Electors are those members of the College of Cardinals who have not reached their 80th Birthday on the day on  which a Pope dies. That is, if they turn 80 years of age before the Pope dies, or on the day he dies, 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 they are eligible. Interestingly, even if they cannot vote they can be elected.

The Cardinal Electors of the Holy Roman Church

As of Sunday, November 08, 2009, there are 185 Cardinals,
113 of whom are eligible to vote and 72 of whom are ineligible.

In the Conclave of April 2005, 115 Cardinals voted.
Joseph Cardinal Ratzinger was elected Supreme Pontiff on the 4th ballot.