My theory is that everything takes a little longer as you get older, but in very small increments, so you don't really notice it. E.g., getting ready in the morning takes just a few seconds longer each day as you get old and slow down, but you remember it taking a set time, say 30 minutes. Then one day, you're up to 40 minutes, but it still feels like 30 to you. Aggregated across your myriad daily activities, and you're either getting the same amount of things done in a day, but your day is "shorter", or you're just running out of time altogether, because "where did the time go?" So, you perceive that time is moving faster, because you're getting less and less done in the same span.