Our perception of time changes as we get older. This is obvious. The simplest way to put this is to say that as you get older, time goes quicker. I find this somewhat true, but I think that the real answer description of our changing understanding of time is a little more complicated. I, however, am only 23 years old, so there is a good chance the issue is even more complicated than this.
I believe that people have two basic conceptions of time: time is the principle that stops everything from happening at once, but that the various events that make up time could actually be switched around with no difference. After all, I can do my laundry this morning or go to the park this afternoon, or vice versa. On the other hand, time also only flows one way, and it is directional. Whatever this direction is on a metaphysical or physical level, the way people understand it is as a narrative progression, a narrative that tells the stories of their lives from childhood to first love to the prime of their life to the inevitable conclusion, death. In this second sense, the events are not at all interchangable.
When you are a child, and even a teenager, these two different scales of time don't really interefere. When I was young, I would see my friends every day at school, and my plans usually ran into the realms of weeks or months. If I wanted to do something, I would choose a different time that week or month to do it. After all, everything would be the same in that time, and thus I could use time to casually arrange my schedule.
When you are an adult, you still have that "casual time" to play around with, but it grows into longer and longer periods of time. Right now, I would say my "casual time" is approximately three months to a year or so. That is, I might think: "hmmm, I haven't been to the Oregon Coast for a while, I should go one of these days". And "one of these days" could be sometime next summer.
The problem with this conception is that "one year", which may seem like a casual amount of time, is actually a fairly large percentage of my total lifespan. I am thinking of going next year as if it will be the same as next year, but actually it will not be the same. Of course, in this case, it is not that large of a deal. After all, I expect to be both alive and mobile enough next year that a walk on the beach will still be possible.
But for some things, I can't just casually throw my casual time around, because it is affected by the overall narrative flow of time. This June, my father told me that we should go up and visit my grandpa's house some day soon. I agreed, but since I was having a bit of a busy summer, I kind of just filed it back in my "do it later" list. Unfortunatly, later this summer, my grandfather got cancer and passed away. It was a reminder of something I knew intellectually, but wasn't really dealing with yet: that I can't just casually put things off and expect to always get back to them, because the time in which I decide whether to finally get around to cleaning out my closet and putting my photos in an album is the same time that brings people through irreversible events, inlcuding their ultimate conclusion.
The same time that you live in, that you work and play and worry about the weather and get surprise packages in the mail is the same time that you, and those around you, will die in.