Why does time seem to speed up as you age?

Two explanations do most of the work, and they complement each other. The first is about memory density: your sense of how long a period lasted in retrospect depends heavily on how much distinct material you can recall from it. Childhood and early adulthood are packed with firsts, new places, new skills, new relationships, and the brain encodes novel experience in detail, so those years feel long looking back. Adult life settles into routine, and repeated similar days get compressed into a generalized memory rather than stored individually, so a year of familiar commutes and similar weeks leaves less to recall and feels like it went fast.

The second is proportional, an idea usually credited to nineteenth-century psychologist Paul Janet: each successive year is a smaller fraction of the life you have already lived, so a year at age five is a fifth of your existence while a year at fifty is a fiftieth, and the relative weight shrinks. Some researchers add physiological accounts, that the rate of processing and encoding images slows with age, effectively lowering the frame rate of experience. The practical implication follows directly from the memory-density explanation and is one of the more useful findings in this area: deliberately introducing novelty, new routes, new skills, travel, unfamiliar people, creates more distinct memories and measurably stretches the felt length of a period in retrospect. Routine is efficient and it is also what makes years disappear.