Lara is trying to remember events from her life as an 18-month-old. however, as hard as she might try, she has no conscious memory for anything that occurred before her third birthday. this is most likely due to the fact that her which is involved in storing explicit memories, was not fully developed at that age.

hippocampus

Explanation:

Humans have two hippocampi which are present on each side of brain. The hippocampus is the part of limbic system and it always plays important role in the consolidation of the information which ranges from the short-term memory to the long-term memory and also in the spatial memory which enables navigation.  

This part is responsible for storing explicit memories and thus can not be developed at early stage and thus she does not remember the things that before third birthday.