What was the significance of the promise that Avraham "would join his forefathers in peace"?
Rashi: Hashem was telling him that he would live a peaceful life, and that he would not suffer any of the travails which He had said earlier his children would suffer. 1
Seeing as Terach was an idolater, how could Hashem promise Avraham that he would join him after he died?
Rashi: This teaches us that Terach did Teshuvah before he died. 1
What was the significance of "the good old age" that Hashem refers to?
Rashi: This was an assurance, a. that Yishmael would Teshuvah in his lifetime; and b. that Esav would not sin publicly as long as he (Avraham) was alive. Indeed, Avraham died five years before his time, so that he should not see that happen, and Eisav sinned on the day that his grandfather died.