"Fifteen." Although she must have known, or guessed, McGonagall still looks surprised at this. "I mean honestly, it’s not all that difficult, at the end of the day. Just gotta be brave, and patient, and determined. You just gotta do all the steps right, and not mess up or you’ll have to do everything again. Peter didn’t really want to do it, almost chickened out a bunch of times; you’re right about that. But James and I convinced him in the end, and we were able to walk him through the transformation. James and I managed it together, first, and I think he was afraid of being left out."