Arenvald definetly grew up in Ala Mhigo. He eventually joined a faction of the Resistance that was trying to get over/under Balsaur's Wall and into Eorzea. They succeeded (around 1.0 I think, it's heavily implied it was during the same incident where Yda was killed). He started getting Echo visions which eventually led him to the Scions which he says was the best thing that ever happened to him. He specifically says that he isn't staying in Ala Mhigo because of all the shit he went though there, and most of it he attributes to native Ala Mhigans...