Another hunch that I have is that as you get older you tend to have less unique experiences. I believe our brains compress the information and an entire day that was much like the previous day can just feel like it sped by quickly. Alternatively, if you go on vacation or change your routine then time will slow down as you process a lot of new information.
That's my theory too. In your first 20-30 years there are so many new things to keep your mind busy. Different schools, driver's license, first car, first time seeing a movie, first time seeing a concert, first relationships, first break up, moving out, travel, first job, children and so on. At some point it all becomes a repeat. I feel a lot of movies are just a repeat of what I have seen 30 years ago. Same for musi…
Even better, an activity where the community surrounding it is also vastly different than your typical friends circle. Know a bunch of engineers? Dive into painting or sculpting.
Find a way to twist your brain in a painful new direction.