The older a person gets, every fixed amount of time, say an hour, a day, a year, etc, is smaller and smaller compared to your whole life. For example, when you are 2 years old, one year is half your life, but it's only 1% when you are 100.

Your brain stores memories more frequently when experiencing something novel. This can make you feel that a longer time has passed when you are doing a lot of new things compared to when you are doing the same every day.