Complex sentence: He celebrated his 60th birthday as a priest on 31 May 1917 and fewer than five years later, on 6 February 1922, he was elected Pope, succeeding Pope Benedict XV, who was only thirty months older and thus from the same generation as Ratti.
Split into simpler sentences: He celebrated his 60th birthday as a priest on 31 May 1917. Fewer than five years later, on 6 February 1922, he was elected Pope, succeeding Pope Benedict XV, who was only thirty months older and thus from the same generation as Ratti.