Rashbam on Genesis 18:12:1:
עדנה - יתעדן הבשר ויתפשטו הקמטין.
Sefaria translation:
עדנה, a form of the skin becoming elastic and the wrinkles
straightening out.
This concept is supported by
Radak on Genesis 20:2:1 (Sefaria English translation) excerpt:
והענין להפקיר אשתו מבלי לעמד בנסיון הריגת עצמו פרשנו למעלה בדבר שרה עם
פרעה. ומן התימה איך היתה שרה כל כך יפה עדיין עד שהיו חומדים אותם לרוב
יפיה והיא בת תשעים, ונאמר כי שבה לעדנתה ולדרך נשים כדי שתתעבר, כי אחר
המעשה הזה אמר וה' פקד את שרה.
ויאמר...אל שרה אשתו
What is more surprising is the fact that Sarah, so many years after
her experience in Egypt, was still so physically attractive that she
inspired physical passions among the people who saw her. Clearly, her
good looks must have been the result of her regaining her youthful
vigour when she became pregnant at the time the angels visited
Avraham, and she herself remarked on that phenomenon. The time frame
in which the events reported in this chapter occurred must have been
immediately after the beginning of her pregnancy, mentioned in 21,1 in
the past tense, i.e. וה' פקד את שרה, “and the Lord had benevolently
remembered Sarah (allowing her to become pregnant).
So, not only do we see that Sarah was attractive, but Sarah herself points that out about herself. So the verses are not at all contradictory.