It is all in the math.
Take a five year old.
One year of that person's life = 20% of his/hers entire existence.
Pretty huge chunk.
Now, take a 50 year older.
That same year is only 2% of his/her entire life.
2 cents on the dollar!
Take the percentages, mix them up with the predictable nature of much of our existence, and the fact that time "flies" is a no-brainer.
One way to "slow it down" would be to either do ABSOLUTELY nothing, ever (watch the dot on the white wall).
Or, try to do new and interesting things as much as you can stand it.
Or, enjoy the moment, now it is a precious gift, and don't waste it on laments.
You choose. Really.