Why did Eliezer mention the fact that Sarah bore Yitzchak only in her old age?
Seforno: To inform them that from birth he was dear [to his parents 1 , thereby boosting the Chashivus of the Shiduch in their eyes].
Ha'Emek Davar: Do not say that Sarah's son is too old for Nachor's granddaughter. Sarah gave birth to him in her old age, so he is not so much older than her. 2
Malbim: He was dear to his father, because he was born through a miracle.
Since Sarah was Nachor's niece, why should we think that her son is too old for Nachor's granddaughter? Rather, Besu'el knew the age of Sarah (his first cousin) relative to himself, i.e. she was born 130 years ago. He could have expected Yitzchak to be about 100 years old! However, if Besu'el's family was in touch with Avraham (or Terach) even after they went to Charan (11:31), until Avraham went to Eretz Kena'an, Besu'el would have known that Yitzchak is less than 55 years old. (PF)
Why did he repeat "Eshes Adoni Ven lA'doni"?
Malbim: He taught that (a) Yitzchak was from Avraham's wife, and not from a concubine. (b) Yitzchak takes after his father in beauty, Chachmah and good Midos.