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For many people time seems to run faster as they get older. That is, for an old person an hour, a day or a year seems to be shorter than for a young person.
Is there any hard data for such phenomenon?
And if so, what is the cause?
To name a few possibilities it can be about the reaction time (so one have more or less 'turns' in a minute), the relative time comparing to one's age, ways in which memory changes with age or habits (younger may live more active life). Of course, there may be different causes for different time scales. However, I haven't found anything but anecdotal data and home-made theories.
There is "hard evidence" regarding how timing and the subjective experience of intervals changes as a function of age. McAuley et al. (2006) ran a battery of different timing and time perception tasks on participants of ages ranging from 4 to 95 years. One finding that relates to your question was that children in the range of 4 to 7 years preferred and, when asked to tap, produced sequences where the inter response interval was much shorter (around 300 ms) than that for adults (around 500 ms). One interpretation of this finding is indeed that small children prefer living on a faster time scale than grownups.
The time of our lives: Life span development of timing and event tracking.
McAuley, J. Devin;Jones, Mari Riess;Holub, Shayla;Johnston, Heather M.;Miller, Nathaniel S.
Journal of Experimental Psychology: General, Vol 135(3), Aug 2006, 348-367. doi: 10.1037/0096-34220.127.116.118, free pdf
The Wikipedia article on "time perception". In particular, the section on long-term time perception cites a couple of articles.
* The articles makes the common point that a unit of time as a proportion of one's life decreases with age. A few empirical studies are also cited.
Ukraintseva (2001) wrote an article devoted to the topic and makes several interesting points about time perception over aging. E.g.,
Ukraintseva argues the rate of aging decreases with age and that this may have relevance to experience of time.
Ukraintseva also suggests that living may be slower in some sense with age citing examples of slower "metabolic rate, rate of cell proliferation, physical, mental alertness, and the rate of living in general".
Graf and Grondin
Graf and Grondin have an article that reviews the literature on the psychology of time perception and discusses age related differences. However, the article seems to focus more on aspects of performance rather than the subjective experience of time slowing.
They cite a few studies that suggest that in relation to judging time intervals, older adults may be less accurate.
My own random musings (treat them as you wish) suggest that topics such as flow, goals, rhythm of life, and boredom might also be relevant.
To extend on @BenCole comment, an interesting summary of different models of time perceptions can be found in this paper. These models are in a sense more descriptive than the fundamental biological hypothesis mentioned by caseyr547, so you might not be ready to call these "explanations", depending on what you mean by that.
The models attempt to give a precise (functional) form to the extent to which time perception decrease as the "stock" of experienced time increases.
[perception of change from time $t$ to $t'$] = constant * $[\log(t') - \log(t)]$.
Apparently, this model was an attempt to link time perception with the so-called Weber-Fechner law relating the intensity of a perception to the magnitude of some initial stimulus of the same nature (e.g. the perceived increase in the weight of an object depending on the initial weight of the object).
However, researchers seem to have found little evidence of such relationship in the case of time perception (see the aforementioned article and http://link.springer.com/article/10.3758/BF03204158#page-1). So other model have been developed. The first paper describes some of these newer models.
Time perception is active field of study in psychology and neuroscience. How individuals experience time is so subjective and interwoven into the fabric of our being that it cannot be directly explained it can only be tested. Current theories include that of lower dopamine in the aged (it is debated) and maturation of the prefrontal cortex and hippocampus (it is not debated). There are other minor theories which have less evidence for their validity. Emotions are known to affect our perception of time in a temporary way.
I'm 33. I don't feel a bit like time is flying. I have aspergers. On the other hand, the was a YouTube video of a 29 year old who said that when he was a child, he lived in the moment and now, he feels like it's really rushing and he has anxiety about it. I have a tendency to do my own thinking from scratch which naturally buries my past in the sand. My awareness goes the other way. I have more awareness of the recent stuff. My childhood like doesn't stand out at all. Once when I went to the Church of the Transfiguration games night recently for the first time in the past year, it triggered another letting go of prior thought patterns and thinking again from scratch. Using slow thinking that just came to me, I remembered that the past did happen after all and was comparing my current thinking with earlier to see that I had let it go and started again. So my brain constructed the sensation that enough time went by for me to let it go and do it again and so eons of time had passed. After we started practicing social isolation, I adapted to it nice and easily like a young and inexperienced child who interprets what's going on right then from scratch. I'm like not thinking about when it's over. I don't have isolation fatigue a bit.
Once I think in the past year, there was a girl who looked about 6 in a driveway drawing a picture with her mother. At first, I walked past, then I remembered it had said the word thank you and then saw and noticed that it could mean thank you for looking at my picture. So I came back and told them I saw and noticed that and then the mother was glad I had seen and noticed something interesting about it. I indeed was able to because the brain isn't hard wired and instead I do my own thinking from scratch and bury my past in the sand.
Once maybe about 2 months after a pancake supper, I had a dream that it was already the time of the next one and didn't like it because it didn't seem very long. Then when it actually was the time of it, I truly felt like it had been really long. My brain had changed enough so I didn't have a problem anymore. Now it may have been something like 5 years ago. Yet, I don't have a problem now. It's almost like I'm a reincarnation and don't have a problem with feeling like less time has passed because less time actually passed and the question is not how much time has passed but how far back your past goes. When I learned about events in our universe from billions of years ago, I felt like it was a new experience of my own. I guess maybe looked back at the buried in the sand past is no different.
I've sometimes had false memories in dreams where my past went totally differently than it did. One more recent case of a false memory in a dream was a very wierd one. In one dream, I was 70. I had a false memory that my recent past was different than it was and that what was going on now was so long ago that it was very buried in the sand and therefore didn't feel like time was rushing and so had no problem.
I guess it's cause I had already been thinking before about how although you feel like the present is fundamentally different from the past, what you're experiencing now actually is essentially the same as what you were experiencing then as long as you're a person who's brain continues doing its own thinking and burying the past in the sand and it's because you're doing your own thinking and naturally letting it get buried in the sand. If that's still what my brain is doing by 70, then technically, it will be essentially the same experience as what I'm experiencing now even though it won't feel that way because my interpretation of now will be distorted. So my brain was able to construct what it could be like when I'm 70. Maybe by then, the dream of it will be buried in the sand just like I felt like now was buried in the sand during the dream. However, I'm not thinking of this as being about a real existing time.