Why does time seem to speed up as we age?

Several theories explain why summers felt endless as a child but years now race by. The proportional theory notes that at age 5 a year is 20% of your life, but at 50 it is only 2%, so each year feels relatively shorter. The novelty theory adds that new experiences create dense, detailed memories — childhood is full of firsts — while adult routines produce fewer distinct memory markers, so time seems to compress in hindsight. There is also evidence that the brain processes new images more slowly as we age, subtly altering our perception of duration. Seeking novelty can help stretch time again.