It occured to me that he must be older than any elected Pope. After a quick check the oldest Pope to enter office since the 18thC was Benedict XVI at 78 and three days. From Wiki:
Pope Year elected Elected at age Age at death or resignation Length of pontificate
Clement X 1670 79 years, 290 days 86 6 years, 84 days
Alexander VIII 1689 79 years, 177 days 80 1 year, 118 days
Paul IV 1555 78 years, 330 days 83 4 years, 87 days
Clement XII 1730 78 years, 100 days 87 9 years, 209 days
Benedict XVI 2005 78 years, 3 days 85 (resignation) / today 93 7 years, 315 days
John XXIII 1958 76 years, 337 days 81 4 years, 218 days
Innocent XII 1691 76 years, 124 days 85 9 years, 77 days
Callixtus III 1455 76 years, 98 days 79 3 years, 120 days
Francis 2013 76 years, 86 days Current / today 84 ≥ 7 years, 296 days
Benedict XIII 1724 75 years, 91 days 81 5 years, 268 days
So my hunch was wrong by a couple of years. Britain has an significantly older head of state though.