As a middle-aged human I first read "How the Fast Days are getting longer" as some kind of ironic commentary with the actual meaning being "how fast the days are getting shorter".
I think it speeds up because we settle in a routine, and when every day is mostly the same the brain just compresses the experience. The younger you are, the more new everything is around you, and you may not know yet what is worth trying or not, hence more things happen, and that makes it appear to last longer.
