It's funny that the book states Sagawa got aiki at 17. Then one reads a bit further on that sometime in Sagawa's adult life, he was trying to figure out how to be "sticky". He worked with women specifically on that. And to read that he was working on it because Takeda could do it.
So, sometime in Sagawa's adult life, he's still working on how to do things Takeda could do. Being "sticky" is part of having aiki.