Sentence:  Although at the commencement of the voyage I had been in bad health, and was at all times of a delicate constitution, I suffered less than any of us, being much less reduced in frame, and retaining my powers of mind in a surprising degree, while the rest were completely prostrated in intellect, and seemed to be brought to a species of <WSD>second childhood</WSD>, generally simpering in their expressions, with idiotic smiles, and uttering the most absurd platitudes. 
Meaning of the word marked with <WSD> tags:  A childlike state in any adult, resulting from mental illness, trauma, or other conditions.