The way i've always understood and the main factor is this:
As you get older the time frame references that you are brought up with eg, hours, days months , years etc represent a smaller percentage of your entire life.
If for example you are ten years old then one year is a larger percentage of your life than if you are 50 years old.
so at fifty, one year being a smaller percentage and you have lived through many of them they seem to go quicker because you see them as small chunks rather than big chunks, and they are percieved as having gone by quicker.
Its just a trick of the mind really based on how you percieve time