"And I am old." From here [we learn] that the Holy One, blessed be He, altered [the words] for the sake of peace. She said, "and my lord is old" (Genesis 18:12), but the Holy One, blessed be He, said, "and I am old." If so, why was the Holy One, blessed be He, displeased with Sarah for laughing? For behold, regarding Abraham it is written, "and he laughed" (Genesis 17:17), yet the Holy One, blessed be He, was not displeased. Because it is written regarding Sarah, "within herself" (Genesis 18:12), but regarding Abraham it is not written "within himself." And what is [the meaning of] "within herself"? She was gazing into her innards and saying, "Breasts that have shriveled — do they draw forth milk?" Therefore the Holy One, blessed be He, was displeased with her.
Why God Changed Sarah's Words and Faulted Her Laughter
Midrash Aggadah, Genesis 18:13
ואני זקנתי. מכאן ששינה הקב"ה מפני השלום, היא אמרה ואדוני זקן, והקב"ה אמר ואי זקנתי, א"כ למה הקפיד הקב"ה על שרה על שצחקה, והלא באברהם כתיב ויצחק, ולא הקפיד הקב"ה, לפי שכתוב בשרה בקרבה, ובאברהם לא כתיב בקרבו. ומהו בקרבה, מבטת היתה במעיה ואומרת שדים שצמקו מושכים חלב, לפיכך הקפיד הקב"ה עליה: