Well when she lost her memories, she lost a little over one years worth, this was when he was Umm… a year before the story starts so she was about 20 when it happened. She stopped working with the company at 18 so she has all her memories prior. She remembers one year of living alone, so she still remembered being friends with Alex. Not that alex would’ve ever let her forget. Alex stayed with her constantly for almost a month after her memory loss.
Anyway, the actual answer to your question would be from when she was very young. Her first memory is of being attacked by a squirrel that she was trying to catch when she was a kid.