One leading explanation is proportional: a year is 1/10th of a 10-year-old's life but only 1/40th of a 40-year-old's, so it registers as a smaller slice of lived experience. Another is novelty: childhood is packed with first-time experiences that the brain encodes in rich detail, making that period feel like it contained "more," while adult routines generate fewer new memories to anchor time to — so in hindsight, repetitive stretches compress into a blur.