Why does time seem to pass faster as you get older?

It is a genuine and widely reported perception, and several explanations contribute. None of them involves time actually changing.

Proportional theory is the oldest explanation, usually credited to Paul Janet in the nineteenth century. Each year is a smaller fraction of the life you have already lived. At five, a year is a fifth of your existence; at fifty, it is a fiftieth. If we judge duration relative to accumulated experience, later years should feel proportionally shorter — and they do.

Novelty and memory density is probably the stronger explanation. We estimate elapsed time retrospectively by how much we remember of it. Childhood and early adulthood are dense with first experiences, each of which is encoded in detail. Later life contains more routine, and routine is compressed in memory — a commute you have made a thousand times leaves almost no trace. A holiday feels long in retrospect and a familiar week feels like nothing, which is the same effect at small scale.

This leads to a testable prediction that seems to hold: periods containing novel experiences feel longer in hindsight, even if they felt ordinary at the time.

Physiological rate has some support too. Some researchers argue that the rate at which we process perceptual images slows with age — fewer mental "frames" per unit of real time — so more real time passes per subjective unit.

The practical implication is the interesting part. If memory density drives the sense of duration, then deliberately seeking unfamiliar experiences — new places, new skills, changed routines — should measurably slow the retrospective sense of time passing.