Trent started showing signs of developing an off switch right around 7 or 8 months old.
For me, an off switch doesn't mean that the dog settles down after hours and hours of exercise. Rather, I see it as a dog that can be able to easily settle even if he still has plenty of energy to burn at the moment. It doesn't happen often, but Trent can go days without exercise and still spend every moment lying down quietly besides me. It's not a matter of not having an "on" switch for him, because this dog is high energy, high drive, and plenty of intensity. But when he was injured (external) and ordered to rest, he was able to go 2 weeks with very minimal exercise (slow, short walks, some obedience and mental stimulation every so often), with absolutely no whining and no pacing, though once we were outside he made it clear he wanted to GO GO GO.
If we just ran 8 miles on a sunny day and topped it off with obedience class, he settles down and turns off at home. If we have gone several days without exercise because of an injury on his part or my part (has happened once), he settles down just as easily without any problem. But once I say the words "let's go!" or wave around his rope tug, he snaps up and goes crazy. And I absolutely love that about this dog.