Seeing as both Avraham (here) and Sarah (18: 12-13) laughed, why did HaSh-m take Sarah to task , but not Avraham?
Rashi, Targum Onkelus, 1 Ramban, Riva: Because whereas Avraham, who believed firmly in the prophecy that he would have a son, laughed out of pure joy, 2 Sarah laughed because she found the prophecy difficult to believe. 3
Ramban: Onkelus' source is that HaSh-m commanded to keep the name. Surely it was not improper laughter!
Targum Yonasan, in both places, translates the laughing as a laugh of surprise. It is not clear how he explains HaSh-m's different reactions to the two laughs.
Riva: Sarah laughed because she thought that it was a person's Berachah. Avraham knew that HaSh-m said so.
Bearing in mind that the earlier generations bore children as late as five hundred years old (e.g. Noach; see Bereishis 5:32), what was so surprising for Avraham and Sarah to bear children at 100 and 90, respectively?
Rashi: By the days of Avraham, the average lifespan had already shortened, and weakness had fallen upon the world. At their age, they were considered too old to have children.
Avraham already had a son (Yishmael) when he was 86, so what did he mean when he exclaimed, "'hal'Ven Me'ah Shanah Yivaled?"
Seforno: What he meant was that, under normal circumstances, a man of a hundred might well bear a child from a young woman, but not from an older one; and as for a woman of ninety, she could not bear children at all, even from a young man.
Lev Eliyahu (Shemos p. 131): The entire reason Avraham wanted a son was to continue teaching the world HaSh-m's ways. He was concerned lest he not live long enough to train him! He prayed that Yishmael, whom he had already trained for 13 years, would fear HaSh-m enough to be Avraham's spiritual heir. This is why HaSh-m responded, "v'Es Brisi Akim Es Yitzchak," i.e. Yitzchak will be Kadosh from the womb (Rashi to 17:19).
QUESTIONS ON RASHI
Rashi writes: "Although in prior generations, men would father children at 500 years, by the days of Avraham the average lifespan had already shortened..." Mizrachi asks - a) Taken in perspective, Noach fathered children at 500 years out of his lifetime of 950; if so, for Avraham to have a child at 100 out of 175 should not be so surprising in proportion; b) Avraham would later father six children from Keturah at age 140; and Yaakov would likewise father Binyamin at over age 95?
Gur Aryeh: Although Avraham himself was indeed fit to have children at that age, he saw that he was [by natural standards] unable to father children through Sarah. For any particular couple, the initial birth is what tips the natural balance and opens the way for more. It would be a great surprise if despite so many barren years of marriage to Sarah, and at his advanced age [when the body is naturally weakened], she would bear him their first child. To answer Mizrachi's first question, men generally do not father children while they are still growing, but rather after they are fully grown. The younger child-bearing age in the generation of Avraham shows that the generations were progressively weaker [and the proportion to the total lifetime is not relevant]. It would thus be a great surprise if at age 100, Avraham would have the ability to father children through Sarah for the first time.