Complex sentence: It is the Dean's responsibility to summon the conclave to elect a new pope when the previous one dies or resigns, and to preside over the conclave unless he is too old to vote in it.
Split into simpler sentences: It is the Dean's responsibility to summon the conclave for the purposes of electing a new pope following a death or resignation. The Dean presides over the conclave unless he is too old to vote in it.