Why is it that as we age, time seems to race along?

Good point. Seems obvious in hindsight although the article also presents another theory: "the monotony of the events of old age compared with the vividness and novelty of the experiences of youth must also be considered."

Yes, at 35, this is how it seems to me, too.

Plus, the number of milestones and changes that occur when you are younger are much greater then when you are older, and those changes are more novel. You have your first word, your first step, your first day of school, the first time you learn to ride a bike, the whole puberty thing. There is a huge number of changes occurring that you have never experienced before. Plus you have the anticipation of all that is to come, and when you are focused wanting something to be here, it feels like it takes much longer to arrive.

As an adult, I often have days when I become so involved in what I am doing that I lose track of time, poof all of a sudden the day is over. The only time I can remember that happening when I was younger was when I was either reading or playing video games.

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.

New experiences are always more vivid. The more unusual it is to your previous experience the move vivid it becomes in your mind.

Take for instance travel. Driving someplace new you're more alert about the course since you don't know what to expect. Once you've repeated it a few times or hundreds of times, like say driving to work, it becomes predictable. As long as traffic is moving along at a decent pace, you're perfectly oblivious to the passage of time.

Now take skydiving or bungee jumping for the first time as another example. You may free fall for 5-30 seconds, but the adrenaline pumping in your brain makes it seem like slow motion. You remember every vivid detail. After 1000 jumps it's nothing near the vividness of the first 10.

The only lesson I've been able to draw from this is that if you want life to slow down you have to live it and experience new things.