When a child is two years old, 1 year is really half of his/her life,
when a person is 20 years old, 1 year is a 20th of his/her life. (and 10 is half, etc etc)
So, when one is younger, 'a day' is actually longer and as one gets older 'a day' is shorter, along with the years.
recapturing of the youth