Time does not actually speed up but our remembered experience of it does. The difference lies between present time which always ticks at the same pace and remembered time which expands or contracts based on how many detailed memories we encode.

More memories make any period feel longer in retrospect. The brain however is wired for efficiency. It defaults to routines habits and familiar patterns to conserve energy. Once something becomes routine it requires minimal attention and creates almost no new episodic memories. This is why childhood feels vast with endless firsts while adult years can blur together.