We are really comparing "50-year-old's current experience" against "50-year-old's memory of 5-year-old's experience". So I've never been sure frame of reference

We are really comparing "50-year-old's current experience" against "50-year-old's memory of 5-year-old's experience". So I've never been sure frame of reference mattered. Instead, I've always suspected it is novelty and memory. To a 5-year-old almost everything is new and novel, and his memory is absorbing absurd amounts of information each day. If he finds, say, a bottle opener and has never seen one before, it might fascinate him. He might spend 30 minutes just playing with it, turning it in his hand, pondering what it is for, etc. Then he'll remember all the little lessons he learned from it. (He might even think more about it later.) You, on the other hand, have seen so many bottle openers that you've got a sort of platonic ideal of it inside your mind. You see it and don't even have to consciously think about it. You might not even remember later that you saw it, where you saw it, what color it was, or anything about it. So my take has always been along the lines that a 5-year-old's experience is filled with brand new interactions with the world, so each day in memory is 'bigger'. We older folks have been there, done that, so the list of things we find important enough to remember is a lot smaller. Most of our "time" is just gone.