(There you go, a bonus blog to make up for yesterday. Apologies to Neal Stephenson for nicking his writing style for this entire piece. This is what happens when you write trilogies of 500-page books, Steve)
It's often remarked that time speeds up as you get older. People always say this as if it's some unknowable aspect of life, but actually the explanation is not only known, but also really simple. Time doesn't speed up, but your memory of time passing does.
Your brain is a great big information processing device designed to help you survive long enough to pump some sperm into something if you are male, or drop a sprog if you are female. It's phenomenally complicated, and for a sack of grey goo, amazingly good at its job. One of the major side-effects of its sperm-protection ability is that it provides us with memory. This allows us to observe the world based on what we have already seen, and thus react more quickly to everyday occurences like the sun rising and rain being cold, saving expensive think-time for more complex problems, like finding someone who will agree to accept our sperm.
Because memory is also expensive, your brain does it very efficiently, making extensive use of deltas. Take, for instance, a sunrise: a gigantic spherical thermonuclear bomb in a constant state of explosion, contained only by the gravitational force of its own fuel, going off right next door, far enough away not to roast us all instantly, but exactly close enough to be pleasantly warm. That's all stunning, but once you've seen one sunrise, you have, essentially, seen them all: you know it's not going to fall down, and that in a certain amount of time it will go away again, and then rise the next day. Your brain, therefore, does not waste precious goo remembering every sunrise you have ever seen in your life. Instead, having remembered one, it then compares any subsequent sunrises to that one, and only bothers to note the difference, or delta, between any subsequent sunrise and your "base" sunrise. So while you may remember a few exceptional sunrises, the majority will just be remembered as a generic "I saw a sunrise" pointing back to that original memory of what a sunrise looks like.
And it is this habit of remembering only differences that produces the strange effects of time that we so often remark upon. When we are children, practically everything is new to us. Therefore, the brain lays down a lot of new memories. Our first taste of jam, our first day of school, our birthday parties, our bicycles: all of these things are radically different to our previous experiences when we are five years old, so our brain spends some time on them.
But as you get older, the proportion of new things that you run into every year gets rapidly smaller. By the time you're in your mid-twenties, in fact, the only time something genuinely new happens to you is when there's a major upheaval in life: you get a new job, or a new partner, or a new house, or you go on a vacation to somewhere you've never been before. These events are months apart.
Where the fault lies is that our perception of our memories is that they are linear -- i.e., in any two periods of time of equal length, there should be an equal number of memories. This is because, perversely, our brain does not understand its own manner of operation. Our memories are not at all linear. When we are five years old, the time for five brand new memories to accumulate is perhaps a few weeks. When we are twenty-five years old, that many new things might not happen in a whole year. So to us, it seems like time is passing more quickly.
It also explains other effects. When one moves into a new house -- as I have just done, which is why I thought to blog about this phenomenon -- one often remarks, after only a day or two, that "it seems like we've been here forever, doesn't it?" This is sometimes taken as proof that the house is a good one. But actually it's just because you're in a new location: lots of new rooms, furniture, streets, shops and public transport links to remember, so your perception of all these memories makes it seem like a lot of time has passed. The same effect happens on vacations to exotic locations: lots of new experiences make the vacation seem longer, so you feel you had a great time. But remembering this, next year you go again and do all the same stuff, and you remark "the time just flew by!" because your brain has seen this all before.
So looked at one way, time stays constant, and it is your perception of it is what is changing. But if you're going to get really metaphysical about things, what is time but our perception that things in the universe happen one after the other? We cannot observe the past or the future: therefore, time exists only in our memories. Therefore, in another sense, time does speed up and slow down, all the time, at different rates for everyone.
So if you feel like life is rushing past you, do something entirely new: the world will skid to a halt, and you can jump back on.