Sentence:  "Book 1": / Reason alone teaches us to know good and bad. . . . Before the <WSD>age of reason</WSD> we do good and bad without knowing it, and there is no morality in our actions. . . . A child wants to upset everything he sees; he smashes, breaks everything he can reach. / "Book 4": / We hold that no child who dies before the <WSD>age of reason</WSD> will be deprived of eternal happiness. . . . The whole difference I see here between you and me is that you claim that children have this capacity [i.e., to recognize the divinity] at seven, and I do not even accord it to them at fifteen. 
Meaning of the word marked with <WSD> tags:  Seven years of age, at which age a person is morally liable for the sins that he or she commits.