From Dave Munger’s “Why do we forget our childhood?“:
… [Freud] did discover an important phenomenon which continues to be investigated today. Freud noted that adults do not remember childhood events occurring before they were as old as six. This period of childhood amnesia is now generally believed to end at about age three or four. Though current psychologists don’t put much stock in Freud’s explanation of the phenomenon (he believed the memories were repressed because they are too traumatic), there is still little agreement on what causes it.
Gabrielle Simcock and Harlene Hayne of the University of Otago noticed that the period of amnesia tends to end at about the time of the onset of language, so they devised an experiment to test whether language ability might be at the root of the problem (“Breaking the Barrier? Children Fail to Translate Their Preverbal Memories Into Language,” Psychological Science, 2002).
They created a memorable event for toddlers of ages ranging from two to three: a magical shrinking machine. …
Six months to a year later, the toddlers were revisited and asked about the experience. Most kids, regardless of their age, could say very little about the shrinking machine. However, when they were shown photos of the toys from the experiment along with decoys (for example, four teddy bears, only one of which was used in the game), they accurately identified the toys from the game most of the time. … The memory existed, but the words were not associated with the memory.
Simcock and Hayne argue that these memories simply are not ever encoded in language, and for that reason, never become part of an adult’s autobiographical memory.