What do you think Martin meant when he said if a 12 year old had to conquer the world so be it?
It means he has more planned for Arya Stark than
any most of us realize. She’s truly the one character this can apply to if we assume (and safely, at that) that each book constitutes one year gone by. Arya’s around eleven years old in her Mercy teaser, and even though time goes by differently in Braavos, by the time she goes back home, she will be considered twelve. He said this in response to being asked about the five-year gap he had originally planned.
Q: 5-year gap?
A: It worked for characters like Arya and Dany but not so much for the adults or those who had a lot of action coming. […] So he ended up dropping it. He said he would have done it sooner if he hadn’t told so many fans about it. And there is no gap anymore. “If a twelve-year old has to conquer the world, then so be it.” (x)
Of the two characters he referenced, Arya is the only one eligible. Dany’s been past twelve for a long time. And it’s noteworthy that almost every single time the gap is brought up, he mentions Arya. This infers that she was a major factor in his planning of the five-year time jump, if not the factor. He could have wanted this gap so either A) she could be past puberty and at the “woman grown” stage (i.e. for sexual situations), B) she would have five years worth of honing her multitude of Faceless Men skills (languages, mummery, cunning, murdering), or both. I tend to think both given that he has explicitly stated that the gap made sense for her training, and the Mercy chapter is Arya’s opener in The Winds of Winter and he has written her as a straight-up femme fatale.
So how can she save the world? Well, to ponder that, you have to look at Arya’s skillset and the common thread weaved throughout her story: assassin-in-training, polyglot, exceptional skinchanger, leader/queen references, and Nymeria, the direwolf whose soul Arya shares that acts as the alpha of a massive wolfpack waiting for their girl in the riverlands. GRRM has poignantly named Nymeria’s pack a Chekov’s gun, and has refused to comment on Arya’s role in future books. Whatever he plans for her, it’s not going to be half-assed or of secondary importance. Arya will be of key significance, and with her wolves, numerous foreshadowing with dragons, and a place in the outline as one of the “key five,” it’s not hard to see why GRRM would be building her up to “conquer the world.”