A popular misconception that needs to be challenged, and one that I fear encourages some non-negligible deal of neglect and abuse.
Children under 4 years old have surprisingly excellent memories, extending years back. But something happens around age 4, possibly a burst of neurogenesis, which scrambles most of whatever episodic or autobiographical memory is in the brain. Other forms of memory endure the transition just fine. Motor skills are obviously retained, as are other life-long skills like how to speak (and for the hyper-literate, how to read). Acquired fears certainly survive, and though they can be overcome... they're still around to actually have to be overcome. Early childhood trauma, and especially abuse, absolutely leave an imprint, even if the events themselves are no longer clearly recalled.