The sentence is “a hundred years and twenty years and seven years”. It is a clumsy way of stating her age. The early chapters of B’reshit often list people’s ages and they do not seem to need such circumlocutions.
The Midrash believes the verse is telling us something deeper, that Sarah at 100 was as free of sin as at 20 and at 20 she was as beautiful as at 7 (Gen. R. 58, quoted by Rashi).
The years were kind to her, and she was kind to her years. She did not besmirch her adult life with sin. She did not lose her sweet appearance during her adolescence.
There is a lesson here. As the siddur states at the beginning of Shacharit, “The soul You placed within me is pure”.
Our aim in life must be to tend the soul and keep it as clean as we possibly can. To reach the end of life, whenever it comes, sour-faced and spiritually pockmarked, is by no means the best we can do.