Lead
Some elementary schools hold a time-capsule ceremony: children write "a letter to my future self" and seal it away. The fact that children write those letters with such care suggests they have an intuition that their future selves will want to know what their present selves were like. That intuition is, in fact, well supported by the research.
The study of autobiographical memory has been quietly clarifying what the act of recording does to memory and to the development of self. And those findings explain why the period around age 8 works as a natural first threshold for handing a child a diary of her own.