This week's Parsha begins by telling us the age Sarah Imeinu was when she died. The Pasuk breaks up each digit in the number of years she lived, with the word "years", and Rashi quotes Chazal who say that this is to tell us that Sarah was as pure from sin at the age of a hundred as she was when she was twenty, and as beautiful at the age of twenty as she was when she was seven.
Rav S. R. Hirsch ZT"L says that the simple reading of the Pasuk reads that Sarah was one hundred years, twenty years and seven years. These three figures correspond to three different periods in a person's life: childhood, adolescence and old age. Sarah used each of these to the full and took whatever she achieved in one period to the next. Thus, she took her childlike beauty into adolescence, and her spiritual purity till her old age.
Similarly later on, the pasuk describes the old age of Avraham Avinu as "ba bayamim", meaning that he "lived the days". He did not let himself drown in the time. He lived each day to its fullest, and took the achievement of each day with him into the next.
Rav Hirsch comments on how generally people live their lives exactly the opposite. Adults wish for the beauty of a child, and grownups wish for the purity of "under 20's". That is not like Sarah, who took the purity and beauty with her into the next period.
The phrase "childlike innocence" is a misconception, because it implies that purity from sin is the same as innocence. This is a mistake. The definition of "innocence" is one who has not been exposed to a temptation, while purity is one who withstood the temptation despite the exposure. Sarah did not live her life in childlike innocence; she took the purity of an "under twenty" with her for her entire life.