I think it was a gradual thing. I think you can place Morrison & Janin's story in a few different places and they work, but the generally chronology is that PKJ's opening arc gave Superman radiation poisoning which led to his weakened and gray-haired state. This prompted him to recruit the Authority to replicate his powers. From that point on, they've been secretly making Clark appear to be at full power even as he's continuing to grow weaker. That's really all we as readers need to know.