There are several reasons. When we are older we are more aware of what is planned in the future. We have a doctor’s appointment next month, or a vacation planned in two months from now, or our mom is coming to visit over Thanksgiving. Children generally are only in the moment. For them time almost stands still while adults are always moving from past, to present to future in their thoughts. A lot of adults are basically waiting for the next break. The next weekend to roll around, the next vacation week, the next happy event, again not in the moment except for brief periods of time throughout the day and even year.
Also, adults have daily routines that blend the days together. The majority of an adult day is usually being at work 10 hours a day if we include commute time, and then surrounding that is waking up, getting read for work, getting home, unwinding a few hours and going to bed. The days pass in sort of a mindless hypnotic state in a way, and then all of a sudden a month has gone by, and then next you know it is already Halloween again and Christmas decorations are being put up in the stores.
The slowest years for me as an adult were the years I suffered with the most chronic pain. Almost every day was difficult, dissappointing, and had some sadness or anxiety. Time dragged. I couldn’t live unaware, I was aware of my day, my present, because of the pain. In retrospect those years blend into a blur, but during those years I felt almost every minute of the day as the minutes went by.