shnei chayei Sarah - the years of Sarah's life
The unusual way about describing Sarah's lifetime is only the repetition of the words shnei chayei Sarah (the years of Sarah's life). From these words our Sages derive the message that all her years were lived in the same state of piety.
"At 100 she was as free from sin as at 20; at 20 years of age she was as beautiful as at seven years of age." (Bereishit Rabbah 58:1) The meaning of these words is that just as a girl's looks are apt to improve after she has reached the age of seven, Sarah's looks kept improving even after she had reached the age of twenty (which is usually the time when physical beauty is at its best). Man's life is divided into three periods: youth, i.e. the period when one grows towards maturity. Maturity, roughly the period between 20-30 before one's physique begins to decline. Old age, a period when decline becomes visible.