That old saw about "time speeds up as you get older" is actually true (in a subjective sense, of course), but not for the reason that most people think (which is any unit of time becomes an ever-smaller fraction of life experience).
While that is arithmetically true, it's just coincidence.
The actual reason is something truly fascinating.
In an entropic universe, the Key to Life is energy: 1) Acquiring it (consume other energy-bearing things, a.k.a. life forms), and 2) Avoid expending it whenever possible.
It is so fundamental that the second part resulted in a highly evolved---and completely subconscious---response of "take no action unless it's necessary to do otherwise" regarding everything.
How it works is the first neurological step of processing experience is separating things that have been done before from things that are new. Like a little traffic cop in your brain standing at the fork of a road. Familiar goes this way, new goes that way.
And only new experiences get the Full Brain Treatment---enhanced senses, awareness, etc.---while the been-done-before stuff just gets a box check.
It's why the days and weeks "run together" when doing repetitive tasks and living the same daily life; while after returning from a three week trip to Patagonia or cruise to Antarctica it will feel like you were gone for months.
Where Mother Nature's grand energy saving mechanism becomes a bummer is we were never DESIGNED to live past the age where our own children can have kids... and the older you get the more likely you've experienced something before. Add to that steadily declining physical ability to Do Stuff---new or otherwise---as you age, and you have a perfect storm.
The good news is simply being aware of all this contains its own solution.