Board Thread:General Discussion/@comment-4906037-20140303092553/@comment-5553481-20140605174130

Considering he was being trained by his grandfather and not his mother, we could assume he began training after the Auswahlen (and his mother's death)... Maybe the grandfather knew when it was going to happen, so he/they kept Uryuu's powers dormant until then?

Side note, according to wikia, this was Yhwach's explanation: (To Uryū Ishida) "It pleases me that you are not such a fool as to simply accept this decision unquestioningly. However, your judgement is lacking. The reason why I have appointed you as my successor should be simple enough to deduce. You do not see? Then let me ask you this. Uryū. Why are you alive right now? It seems you are quick to understand. Nine years ago, when I carried out my 'Auswählen', all living Gemischt Quincy breathed their last. All but you. You are the only Quincy in all of history to survive my Auswählen. This is why I referred to you as the 'final Quincy to survive in this world'. There is something you possess that surpasses even my own power. That is why I chose to name you as my successor. Understand your place, Uryū. There is no need for doubt. You need only come with me."