Keller means that as you get older, memories are not as strong because more time has passed.
At the beginning of her autobiography, Keller describes the difficulty of writing about her early life. Sometimes some memories are distant, and others are forgotten or misremembered.
When I try to classify my earliest impressions, I find that fact and fancy look alike across the years that link the past with the present. The woman paints the child's experiences in her own fantasy. (Ch. 1)
Keller says that in order to avoid making her biography “tedious” she will focus on the episodes that stand out to her as the most “interesting and important (Ch. 1). The point is that some people’s lives are more interesting than others, but not all parts of even the most interesting person’s life are interesting.
Helen Keller led an interesting life. When she was a toddler, she became very ill, and lost her sight and hearing. Yet she was so intelligent that she managed to learn to read and write with the help of her teacher, Anne Sullivan. Keller shares the pivotal movements that shaped her childhood in her biography. She describes her life from when she was born until the time of writing, as an adult.
The book begins by describing her home and parents, then describes her early childhood and teacher, and then her education. Despite what Keller comments about memory being faulty, her descriptions of even early childhood are very specific. She can describe how she felt, even as a child. Consider this description of her child self struggling with not being able to communicate before she was brought a teacher and only new a few signs.
The few signs I used became less and less adequate, and my failures to make myself understood were invariably followed by outbursts of passion. … I struggled–not that struggling helped matters, but the spirit of resistance was strong within me… (Ch. 3)
Keller’s strength as a writer is her ability to help the reader imagine events, because she can describe them in such vivid detail and include the emotional component. She always describes not just what happened, but how she felt about what happened.