July 20, 2010.
On July 19, 2010, Benedict XVI passed Gregory XIII to become the seventh-oldest pope in history. Benedict XVI turned 83 years, 3 months and 4 days old.
The sixth-oldest pope was John Paul II, living 84 years, 10 months and 15 days. Benedict XVI would pass John Paul II on February 29, 2012, a leap day.
The oldest pope dating back to 1400 was Pope Leo XIII, who lived to 93 years, 4 months and 18 days. Benedict XVI will have to live to September 3, 2020 to surpass Leo XIII.