Sentence:  On September tenth — the morning of my seventh birthday — I came downstairs to the kitchen, where my mother was washing the dishes and my father was… reading the paper or something, and I sort-of presented myself to them in the doorway, and they said “Hey! Happy birthday!” And I said “I'm seven.” And my father smiled and said “Well, you know what "that" means, don't you?” And I said “Yeah… that I'm gonna have a party and a cake and get a lot of presents (?)” And my dad said “Well, yes, but more importantly, being seven means that you've reached the <WSD>age of reason</WSD>, and you're now capable of committing any and all sins against God and man.” 
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.