Perception of time as a function of age 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.
Wikipedia on time perception

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)

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. 


Personal musings

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. 


A feature of being immersed in a state of flow is a sense of time distortion.
As we get older, perhaps we start to set more long term goals, and this might shape the experience of time and change.
Increased levels of routinisation and automaticity might also decrease awareness of time passing.
Perhaps knowing how to avoid boredom would be relevant to time passing.


References


Graf and Grondin "Time perception and time-based prospective memory" in TIMING THE FUTURE - The Case for a Time-Based Prospective Memory (FREE PDF)
Ukraintseva (2001) Aging and the subjective sense of time. (FREE PDF)


The above are just a few quick points (it's not my area):