Most adults can't remember anything that happened before they were 3 years old.
Most adults can remember only one or two events before they were 4 years old and nothing before they were 3 years old. Children begin to have semantic memories (identifying objects in their environment) by 10 to 12 months, and they appear to develop episodic memories (of things that happened earlier in time) by 20 to 24 months. Young children may be unable to form memories solid enough to survive into adulthood until they can think of their experiences in words.
Sleeping is the key who unlocks your undisclosed memories
Studies show that the human sleep cycle is crucial to creating memories. If something important happens to you during the day, your brain strengthens your perceptions of that event overnight while you rest.