Time runs slower when you're young; but then as you age there's less time in each day. Perhaps it's the same for everything else that experiences time, from us to galaxies and universes.
This is because information in your brain is constrained by physics. It takes tangible time for signals to travel between neurons. When you're young, you're small, and the distances are shorter. As you age, you grow in size, and the signal length grows also. Thus as you grow you literally think slower; though hopefully, more effectively (wisdom). Your perception of time changes accordingly.
By age 2 your brain is ~80% of its final adult size, IIRC. So not sure if this idea is correct (but at the very least plausible and intriguing!).
I think the experience of time has something to do with focus. When you are young, or on lsd, your brain doesn't filter out all the signals and so time "slows down" as you suddenly need to process more. I know this is a well researched idea, just forget the actual terms for it.
I always took it as the number of new experiences you’re having. A work week might feel slower than a vacation week in the moment, but vacations tend to be more memorable. It’s shocking how much stuff you can pack into a few memorable days while months blur together.
There is research that shows that subjective passage of time depends on physiological changes to the brain. The most compelling example is that certain brain tumors make the passage of time seem extremely fast or slow to patients with them. The fact that time “seems to go faster” as you age is related to a normal aging related change (though natural, not caused by tumors.)
My flippant guess would be that time perception changing with age would be related to the NMDA receptor. NMDA receptor function decreases with age[0]. My personal anecdotal experience with NMDAR antagonists is that it feels like the `Turbo` mode on old IBM compatibles[1] -- i.e. downclocking. And the slower you process, the faster the world seems to move around you.
From what I remember from development 101, there are also more possible synaptic connections in early childhood, and these get pruned down over time. This would also reduce the amount signals travel.
That's my perception as well, but iirc, time was slow for me when I was 25. I'd attribute this acceleration to neurons wearing out and thus needing more time to conduct signals, as well as growing apathy when there's less "wow" moments in life and more of "I've seen this before". Adrenaline allows to temporarily restore the proper pace of time.