In 'Once Upon A Time' it can be said that in the Control Room, No.2 and the Supervisor work together in order to condition the mind of No.6 to accept that he is No.5. This is demonstrated when No.2 asks the boy Prisoner to recite the even numbers two, four, six, eight go. No.6 begins "Two, four.....five!" No.6 is also prompted by No.2 to say the number six. But so conditioned, No. 6 cannot say the number six, because he's five, that's him!
On the other hand there is another way to look at it. Because there is the regression of No 6's mind back to his childhood, to the age of 5 years. It's as the Supervisor says 5 5 5 5 5 5 5 5 5 5 5 5 5 5, and he holds on 5 so to indelibly imprint his age on No.6'.s mind.