My professor told me he was in his mid thirties, but wouldn't this mean that Dante considered 70 years to be the lifespan of a person in the 13th century?
He started writing it in 1308 at aged 43 and finished the entire collection at age 55 in 1320. Oh and the life span thing refers to the bibical life span see in Psalm 90:10 "The years of our life are seventy, or even by reason of strength eighty; yet their span is but toil and trouble; they are soon gone, and we fly away."