Someone strong in the Force who's grown up not being taught by either side, is something we haven't seen before, at least not on screen. (Even in the Old Republic, the Jedi couldn't possibly find every kid with Force abilities.) The closest thing is Desann in the Jedi Outcast game.
I found the whole "too old to be trained" thing a bit suspect, as those rejected would be easily pickings for the Sith, or just go dark side on their own.