Members of the College of Cardinals who are under the age of 80 are eligible to vote for the Pope.
A cardinal may be retired from his last post, but if he is still under 80, he may vote in the election.
There is a short interval of time between the date the Chair of Peter becomes vacant (sede vacante) and the date on which the conclave begins. Any cardinal who is under the age of 80 at the time of sede vacante may vote, even if he turns 80 before the actual conclave. For example, Cardinal Kasper of Germany turns 80 on March 5, but since he was still 79 at the time of Benedict XVI’s resignation, he is eligible to vote for the new pope.
As of this writing, there are two of the 117 age-eligible cardinals who will not be voting: Cardinal Julius Riyadi Darmaatmadja of Indonesia is excused due to his poor health, and Cardinal Keith O’Brien of Scotland resigned February 25 and announced that he would not attend the conclave. This leaves 115 cardinals to elect the new Pope.
A chart listing all age-eligible cardinals can be found at canonlaw.info.