Here is an interesting bit of information from Catholic Hierarchy:
While doing some research I discovered that my wording on the website relating to the age of a cardinal and his right to vote in a conclave was not accurate. I’m in the process of updating the wording on the site.
The old wording was:
Cardinals under the age of 80 at the time the Holy See becomes vacant may vote in a conclave.
The new wording is:
Cardinals that reach the age of 80 before the Holy See becomes vacant may not vote in a conclave.
It may not seem like that big of a change, but there is one important distinction. [NB:] If the Cardinal turns 80 years old on the same day that the Holy See becomes vacant, he does retain the right to vote in the subsequent conclave.
(Source: #33, Universi Dominici Gregis, by Pope John Paul II, on 22 Feb 1996)
On another note, I believe that if a man is named Cardinal but the Holy Father dies before the consistory in which that Cardinal would receive the marks of his dignity (red hat and ring), the man named Cardinal would be NOT allowed to participate in the conclave. The names must be published in the consistory, not just made public.