I was talking with my grandmother the other day, who is 94 years old. She said something to the effect of "It's just like I eat breakfast and then it's time for bed."
My theory on this phenomenon is that one's memory is like a slide projector, fillled with snapshots of images, sounds, feelings. As an infant, the mind is filling itself with as much information as possible, in order to learn and adapt to a new environment. So the mind takes 10,000 "memory snapshots" in ten minutes, making time appear to go very slow for a child. As we age, less and less of the information our senses process is neccessary for survival, (and we probably have less room for memories or something like that) and so our mind takes fewer snapshots... thus making time appear to go faster. It's like the difference between watching a movie in slow-motion or all speed-ed up: the only difference is the amount of information that the camera recorded.