From Nancy Ruth Jarbadan:

Re: Why we feel “slow motion” during crisis (Dec. 11):

“This is re­lat­ed to the phe­nom­e­non that time seems to speed up as you grow old­er. When you’re a child, you lay down rich mem­o­ries for all your ex­pe­ri­ences; when you’re old­er, you’ve seen it all be­fore and lay down few­er mem­o­ries,” he re­marked. “There­fore, when a child looks back at the end of a sum­mer, it seems to have lasted forev­er; adults think it zoomed by.”

I’ve heard another, similar explanation as to why this is so. As you get older, a “one year” span of time becomes a smaller percentage of your total experience. For example, for a 5-year-old looking back on the past year, that year is 20% of his or her total time experience. Thirty-five years later, for the now 40-year-old looking back on the past year, that year is only 2. 5% or his or her total time experience. Since, as one ages, the passage of each year becomes a smaller percentage of one’s total time experience, a year-in the-life of an adult seems to pass much more quickly than a year-in-the-life of a child.