The former has actually been discussed a LOT here, and Troy is certainly of the mindset that age shouldn’t slow you down, in and of itself. I’ll add, on a personal note, that I caught the John McLaughlin/Jimmy Herring tour a few years back and I have NO idea how old McLaughlin is these days, but he is old by any even conservative definition of the term… and he can still tear it up.
Meanwhile, Satriani is… jesus, 69 years old, per Wikipedia, which is crazy to think about. And I think his last three albums are some of his best playing in years. I’m a veritable spring chicken by comparison at 45, but I think I’ve made more progress on my technique since turning 40 than I did though my entire 30s, by a pretty significant amount.
As far as consistency and having off nights, I think what constitutes an off night changes as your playing improves, and Johnson’s off nights are quite a bit better than yours or mine… but we all have them. Part of the point of locking in efficient technique is to minimize the “cost” of just not having it one night, but there are definitely days where my chops just aren’t really there, and that just happens.