Who will be the new pope to lead more than one billion Catholics will be decided by a group of cardinals with an average age of more than 70.

According to the Catholic faith, the decision is made by the Holy Ghost, which guides the proceedings, and not by the gathered cardinals of the conclave.

While it is not possible to put an age on such an intemporal manifestation as the Holy Ghost, finding data about the clergy coming to the Vatican City can be done more easily.

There are 117 members of the conclave who will gather at the Vatican to deliberate and elect a successor to Pope Benedict XVI by giving one candidate a two-thirds majority.

Voting takes place in the Sistine Chapel beneath the world-famous depictions by Renaissance master Michaelangelo of the story of Genesis and the Last Judgment. Ballots are placed in a chalice on an altar and seniority dictates the order in which cardinals submit their vote.

When deliberations are complete, the world will know there is a new Pope of the Catholic Church when white smoke billows from a temporary chimney on the Vatican roof.

But who are the men who cast their vote in a matter in which one person in every seven on the planet has an interest?

Only five of the members of the group of top members of the Catholic clergy are aged under 60. The oldest will be 80 when the decision is made. A majority (74) of conclave members are older than 70.

Rules forbid a cardinal older than 80 from taking part but Cardinal Walter Casper gets a pass for being 79 on Pope Benedict's last day in the post.

At the other end of the scale is the youngest cardinal, Baselios Cleemis Thottunkal, at 53.

A majority of the 117-strong conclave (57 percent) was appointed by Pope Benedict, with the rest of them owing their position to his predecessor, John Paul II.

They are drawn from 50 countries - more than half of them are from Europe. Catholicism has been shrinking in this traditional heartland of the Roman Church, while rapidly growing in Africa and South America. Yet those continents contribute only a quarter of the total number of cardinals.

Observers say that among the favourite contenders to succeed Benedict are Angelo Scola of Italy, Peter Turkson of Ghana, Marc Ouellet of Canada and Francis Arinze of Nigeria.

Addressing cardinals on his final day in his role, Benedict said: "Among you there is also the future pope to whom I promise my unconditional obedience and reverence.

"The Church is a living being [but it] also remains always the same."