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Supposedly the reason for this perception is because the younger we are, less time means more to us. For example, if I am seven years old, one year will be a seventh of my life. If I am twenty-six, a year is a significantly smaller fraction.
Even on a smaller scale there is some truth to this. I remember my dad taking us to an antique store when we were younger, and it felt like we were in there forever. Now if my dad takes me to an antique store, it may take us half an hour, but it doesn't feel like I'm wasting my life in there. My disposition toward antiquing hasn't really changed, so there has to be some validity to this phenomenon. I don't think it can be proven scientifically, but I know I am not the only one who thinks this way.
Time has been scientifically proven to be relative however. Have a happy new year!