As a corollary, when everything seems old hat to you, you've truly become old.

True wisdom maybe maintaining your curiosity and an open mind so you can branch out at any age but knowing when to "split into new branches" and when to "[merge] into a previous experience". Given how abstract that sentence is, this is clearly an art.

I've always thought it to be the exact opposite. Experience for a child is intermingled because children can often only perceive one plot-line. I remember that, when I was 5, I would watch reruns of various TV shows and think that prior episodes had been taken apart and combined. In fact, they hadn't been, but in my mind the 2 or 3 plots in each episode were stored as separate entities.

When you're older, you're more able to process concurrency and your sense of self is more compartmentalized. You have a work-line consisting of your career and job life, a family-line, a vacation-line that emerges when one travels and is otherwise ignored, and probably several friends-lines (college friends, work friends, friends of the families) corresponding to various distinct social groups. Each plot-line is allocated only a fraction of the time given to the single plot-line a young child has, and thus each progresses much slower per year.

For a child, a year is an eternity. For an adult, it can be very short, because many plot-lines advance very little in this time. If you have a vacation spot you visit every year, every time you go you are bringing up year-old memories that seem as if they occurred yesterday, so time seems to be going extremely fast.