。
It is the poor who died the youngest.
Literal
Poor people [emphasis-こそ] most young died.
こそ is the emphatic particle stressing that the poor, above all others, died young. もっとも is the superlative adverb 'most.' 若く死ぬ uses the adverbial form of 若い with 死ぬ to mean 'die young.' The sentence has a social-commentary or historical tone.