How we know
The proportional theory was published by French philosopher Paul Janet in 1877 in Revue Philosophique. Janet argued that subjective time scales with the proportion of life already lived. A year for a five-year-old is one fifth of their existence. For a fifty-year-old it is one fiftieth. The ratio drops every year, so each year of life feels shorter than the last.
Modern psychology has tested this. William Friedman at Oberlin College has spent decades measuring time perception across ages. His work, summarised in his 2013 chapter in Subjective Time (MIT Press), confirms that older adults consistently judge recent periods as shorter than younger adults do, in ways the proportional theory predicts.
A second mechanism is memory density. Routine experience generates fewer distinct memories than novel experience does. When you were a child, every week brought new firsts: first day of school, first time at the beach, first time you tried a new food. As an adult, weeks blur because the underlying days look like each other. Fewer distinct memories per year makes the year feel shorter in retrospect. This is documented in Wittmann and Lehnhoff 2005, European Psychologist.
A third mechanism, proposed by Adrian Bejan at Duke in his 2019 European Review paper, is image-processing latency. Bejan argues that as neural pathways lengthen and degrade with age, the brain captures fewer mental images per unit of clock time. Fewer mental images per second means subjective time runs faster, the same way a video at a lower frame rate appears sped up. The Bejan model is the newest and least settled of the three.
What it means
Time does not actually move faster. Your reference frame changes. Two practical implications.
First, novelty extends subjective time. A two-week trip to a new country feels longer in memory than two weeks at home, because the trip generates more distinct memories. If you want longer-feeling years, schedule novelty. Travel, new skills, new people, new routes to old places.
Second, the rate of acceleration slows in old age. The biggest perceived shifts happen between five and twenty. After forty, the curve flattens. You are not on a runaway clock.