My theory on time and aging

Everybody pretty much agrees that every year gets shorter. This is my theory as to why. I think the human mind is finite, it has a certain size. A child one year has one year of memories and perception, which fills the entire mind. This makes it seem huge. A child who is two has a year filling up half his mind. For a person who is 40, the year fills up 1/40th of the mind, each year seems small. I'm not saying I think it's precise, a more significant period of time probably occupies more "space" than an insignificant, and the period of a year is defined by humans. (Your brain doesn't know the Earth travels once around the sun in a year, a year is a unit of time you know because your were told) So every year has proportionately has less of a portion of the mind to occupy, hence it seems smaller.