She looked young, but not younger than a lot of the kids we saw training in the prequels. I'm not willing to say I think that hypothesis has any firm ground, but I do think Rey's mysterious origins only further build up her character well beyond the everyman that Luke was and the story (at least lacking the reveal to come) is worse off for it.
The younglings in the Prequels were being trained at the Jedi academy by a Jedi Master, there was none of that on Jakku. Which was pretty much the point I think, whomever left her did it to try and hide her. She knew droid speak and she could understand Chewie. She's like a cross between MacGyver, Doctor Who and Xena, clearly she was meant to hidden for some reason.
There wasn't any of that on Jakku, but she wasn't necessarily always on Jakku. If she was a survivor of Snoke/Ben's massacre as a child she might have been taken away and hidden and whoever was entrusted with her had to leave for some reason and didn't make it back to her.