5. Pope Gregory XIII may not have been the oldest Pope in history, but the octogenarian Pontiff was one of the most influential
Born in 1502, Ugo Boncompagni didn’t even join the Catholic Church until he was 40-years-old. Prior to that, the Italian had studied law and then worked as a professor at the prestigious University of Bologna. But it was this professional experience which meant that, when he finally did take his Holy Orders, his ascent was swift and seemingly unstoppable. Pope Paul III recognized his talents straight away and gave him a series of increasingly-influential positions within the Vatican. Paul IV made him a commissioner for Church reforms and then, at the age of 63, Boncompagni was made a Cardinal.
When he finally made it onto the Papal Throne and named Pope Gregory XIII, he was 73-years-old. While he may not be the oldest man to take on the seat of Saint Peter (Clement X was 79 when he became leader of the Church and, more recently, Benedict XVI was 78 when he was named Pope in 2005), he was probably the most influential of all elderly pontiffs. Above all, Gregory XIII is best remembered for revamping the calendar with the introduction of leap years. The Gregorian Calendar, which he introduced when he was in his 80s, remains the global civil calendar to this day.