After the harsh heave-ho followed by the harsh life on Jakku, she barely remembers anything about her past. It's been suppressed and forgotten, including her force abilities. Or perhaps Luke even did a Jedi mind trick on her to make her forget her youth and training. Until they are unlocked by Kylo Ren trying to delve into her mind. Old force training kicked in to defend her from Kylo's mental assault.
Hell you don't even need any mind tricking to account for that. I can't remember anything beyond about 6-7 years old, only vague snippets of thought and memory. Like the hallway outside of my kindergarten class. Or seeing Return of the Jedi, and even then, it's only vague moments that I remember. Everything else is simply not there. Granted, I could say this about any period of my life, memory of past events isn't something I really devote a lot of memory..(haha!) to in my operating system I call a brain.
I'm 42 and I still remember some things from when I was 4 in vivid detail.