John Gill Commentary Genesis 18:12

"And Sarah laughed within herself, saying, After I am waxed old shall I have pleasure, my lord being old also?" — Genesis 18:12 (ASV)

Therefore Sarah laughed within herself: Not for joy of a son, and as pleased with it, believing so it would be; but as disbelieving it, and perhaps deriding it, and confuting it with a laugh, which, though it did not appear in her countenance, was secretly in her heart: saying, not with her mouth, but in her mind, after I am waxed old, being almost ninety years of age, shall I have pleasure? in conception, or rather in having a son, and in suckling and nursing him, and bringing him up; for in bearing and bringing forth is sorrow: and my Lord being old also; which increased the difficulty and her unbelief: the Apostle Peter seems to have respect to this in (1 Peter 3:6).