Somebody wants a banal explanation?
Here it is:
When you are four, you have lived a total of four years - of which you remember not even that much. The year from your fourth to your fifth birthday represents a period roughly equal to one fourth of all the existence that you can recall, so far. It can seem endless as you live through it.
When you are 54, the year until your 55th birthday represents one-fifty-fourth of your existence - a tiny fraction, by comparison. Years now speed past, with large chunks consumed by the responsibilities of keeping yourself and your family afloat.
When you pass the half-century mark, you can count on your "second half" having a subjective duration of MUCH less than the first half (and less again, since odds are you won't make it to 100). Moreover, this will be a time when your powers are deserting you at an ever-accelerating pace, and the chores of life - those niggling things that take you away from accomplishment - will take up more and more of your remaining time. I look at my 87-year-old mother-in-law taking ten minutes to get her shoes on, an hour to shower, and so on.
Somebody else mentioned the 10,000-year long view. At best, I have 25 usable years left. That leaves 9,975 I will never see. I am reminded of the statement - variously attributed - that the best battle plans never survive first contact with the enemy. In this case, the enemy is time itself, and any plan you might have for the year 12,012 is not going to survive to the end of your life, let alone another hundred centuries.
The evidence suggests that the only thing that I could do now that might be remembered that long (or dug up by future archaeologists - human, or whatever succeeds us) is to start a religion. While I might be a little grumpy, I don't dislike the world enough to inflict another religion on it.
All I can really do to make my life seem worthwhile is to look for ways to be good to other people, right now. If I, or you, get old enough, we won't be remembered more than a few months beyond our expiration dates. For a little while, brief mental images of us might be brought to mind when the living talk about their own past, or when they look at old pictures, but those will be detail-free place-holder "memories", useful more as reference points in their own histories. After a decade or two, even those occasional mentions or reminiscences will evaporate. The file formats of the pictures will no longer be retrievable (think how easy it ISN'T to view and save those old Sony BetaMax tapes that your uncle recorded in the 1970s). Right now, I can conjure up only the fuzziest images of my own mother any younger and more vibrant than the reduced version that she was in the months before her death.
Maybe you can do better, but have you tried? Picture your own Mom now (if she's still alive). Now try to picture her twenty years... thirty years.... fifty years younger, NOT as still photo images, but as a living, talking, acting person when she was that age. Chances are, you have a hard time seeing past your current view of her, without resorting to photos. Picture the viewpoint you had as a five-year-old in the family kitchen. You had to look up to see your Mom's face. In that memory, is her face clear and sharp? Or is it vague and ephemeral - more a representation of the idea of "Mom" - and you have to struggle to impose clarity on it?
You (and I) are no less ephemeral. Be good to somebody now. All you have is the moment, and then maybe the fading memory of it.