[A] popular theory about why time feels different as a child than it does as an adult is the ratio of any given day, week or year to the amount of time we've been alive. To a 5-year-old, a year is 20% of their entire life. For a 50-year-old, a year only is 0.2% of their life, so it feels like it went by much more quickly.
It's also a matter of how much change has happened in that year. A year in the life of a 5-year-old is full of rapid growth and change and learning and development. A year in the life of a 50-year-old probably isn't a whole lot different than when they were 48 or 49. Even if there are major life changes, the middle-aged brain isn't evolving at nearly the same rate as a child. A 50-year-old looking back at the past year will have a lot fewer changes to process than a 5-year-old, therefore the year will seems like it went by a lot faster.
“Our perception of days, weeks, years and that kind of time seems to be especially influenced by our perspective: Are we in the moment experiencing it, or are we looking backward on time?” psychology professor Cindy Lustig told the University of Michigan.
