The Torah lists Sarah's age at her death as being ?One hundred years, and twenty years, and seven years? (Bereshit 23:1). Why was it necessary for the Torah to divide the years of her life into three separate segments? Our Sages reply that in this fashion the Torah indicated to us the following about Sarah:
When she was one hundred, she was as lacking in sins as she was at the age of 20; and when she was twenty, she had the same beauty as she did at the age of seven. This immediately raises the question of how is it possible that a girl aged seven can be more beautiful than one aged twenty? However, the Torah is telling us that just as Sarah did not use her innocent beauty at age seven for anything immoral, so, too, she did not use her more mature beauty at age twenty for anything immoral. (Bereshit 23:1, Rashi).