Actually they have done some research into this and here is the latest theory that I've heard from a neuroscience magazine.
In short, your brain is wired to catalogue and remember new experiences. When you are young, most of what you are doing is learning. Even when you aren't realizing it, you are constantly observing new things, having new interactions, and learning new things about the world and how it works. However, as you get older, you have fewer and fewer "original" experiences so your brain tends to gloss over them, storing them as groups instead of individual moments.
The effect is that during childhood you are remembering and cataloging many more experiences and so times seems to pass slower. It isn't actually passing slower of course, you are just remembering more specific incidents between two points, which skewers your perception of time. As you get older, your brain groups like experiences and you remember fewer instances between two points, so time seems to pass faster.
It's also why people can usually clearly remember events like their wedding, their first day on the job, the birth of their child, etc, because those things are rare and unique events, but they have trouble telling you what happened at work last Thursday because unless something unique happened that day it was just like 1000 other Thursdays at work that they've experienced. The brain just kinda glosses over it as a common experience.
Another common example is driving. Have you ever driven a road so many times (the same route to and from work or the grocery store for years, let's say) that sometimes you get home and honestly don't remember driving the route? That's your brain essentially ignoring the experience because there was nothing novel about it. When you are 16 and learning to drive, you never zone out like that because the roads and driving itself is so new and you are still learning. But at 35, driving is so second nature you often have such "blackout" periods. you don't actually blackout and you aren't unsafe, but after the fact, your brain just ignores the experience.
I hope that makes sense, but basically more unique experiences happen more often when you are young and that leads to more memories and brain pathways being actively formed and that leads to a perception that time is moving slower, but as you age you have fewer unique experiences and so your brain doesn't differentiate them and because you remember fewer unique events it seems as if time is passing faster.
If you want time to pass slower, go experience new things as often as possible.