The "Janne's Law" explains why adults feel that a year goes by faster. This psychological phenomenon was proposed by the 19th-century French philosopher Paul Janne. For example, when you are one year old, one year is the whole of your life (1/1), but when you are fifty, one year is only a small part of your life (1/50). As the proportion of one year in your life decreases, time feels like it is passing more quickly.
Moreover, new experiences are easier to remember and make time feel longer, while familiar behaviors seem to pass quickly. When you are a child, you have many new experiences, so time feels longer. However, as an adult, fresh events decrease, and daily time feels shorter.
For instance, when you go to a new place, the journey feels long, but the return trip feels short because you are familiar with the way. This feeling follows the same principle as Janne's Law.