Having stated above (Bereishis 18:11) that Avraham and Sarah were old, why does the Torah see fit to repeat that Avraham was old?
Rashbam and Seforno #1: Not knowing how long he would live, Avraham decided that it was high time he found a Shiduch for Yitzchak before he died.
Ramban and Seforno #2: It also explains why he made Eliezer swear that he would fulfill his request (lest Avraham die before Eliezer does it) 1 .
Ramban: Here, "Ba ba'Yamim" 2 implies that he was very old. Above, it says Ba'im ba'Yamim" - they were beginning to get old.
Moshav Zekenim #1: It says so after Sarah's death, to teach that a woman dies to her husband, and due to his pain, Avraham aged.
Moshav Zekenim #2: Avraham had returned to his youth, so it needs to say that he was old.
Moshav Zekenim #3: It says in Bava Metzi'a (87a) that until Avraham there was no aging. Avraham requested mercy, and there was aging. 3
Bava Metzi'a, 87a: Until the time of Avraham, nobody ever aged (in looks). Consequently, someone who wanted to speak with Avraham, he would address Yitzchak, and vice-versa (because Yitzchak looked like Avraham. So Avraham prayed for a distinction between them
And he might then find him a wife from Kena'an.
Divrei Eliyahu: It says that he was Zaken and Ba ba'Yamim - he was old, and all his days were full of Torah and Mitzvos. Hadar Zekenim - Ba ba'Yamim hints that he recognized Hashem from the age of three (the Gematriya of Ba).
Moshav Zekenim: Rashi explained there that he requested this, for people could not distinguish him from his son. This is difficult, for before Yitzchak was born, it says that Avraham and Sarah were old (Bereishis 18:11)! (Perhaps there was no weakness due to aging, but normally, children were not born to old people. However, Rashi (Bereishis 18:12) connotes that Sarah had aged! - PF).
By the same token, the Midrash learns from the Pasuk (27:1) "Va'tichhehnah Einav me'Re'os", that, until the time of Yitzchak, nobody suffered Yisurim, and that Yitzchak prayed for them and his Tefilah was answered. It also learns from the Pasuk (48:1) that, until the time of Ya'akov, nobody became ill before dying, and that Ya'akov prayed for it and was answered. See Oznayim la'Torah in Toldos, who explains the reasons behind the Avos' requests.
Since Chazal interpret the word "Zakein", which is the acronym of 'Zeh Kanah Chochmah', to mean 'sage'
What does "ba'Kol" incorporate?
Rashi: ba'Kol refers to a son (Yitzchak), as is hinted in the Gematriya of the word (52, the same as 'Ben').
Targum Yonasan: It incorporates all kinds of blessings (as the word suggests).
Ramban #1: It incorporates riches, property, honor, long life and sons.
Ramban (citing the Gemara in Bava Basra, 16b): The Gemara cites a triple Machlokes Tana'im 1 , as to whether the B'rachah refers to the fact that he had a daughter, that he didn't have a daughter 2 or that he had a daughter called 'Bakol'. 3
Bava Basra (16b): During his lifetime, Eisav did not rebel, or Yishmael did Teshuvah.
Bava Basra, 17a: 'Hashem loaded Avraham with a taste of Olam ha'Ba and neither the Yeitzer ha'Ra nor the Mal'ach ha'Maves nor worms (after his death) had jurisdiction over him'.
Bereishis Rabah 59:7: He ruled over his Yetzer, Yishmael did Teshuvah in his lifetime, and his storehouses did not lack anything. Some say that Hashem did not test him further. 4
Mishnas R. Aharon 3 p.119: He had the Midah of "Kol" 5 - he has everything, and he cannot add to what he has through what is in another's hand, or through getting honor or any other benefit from someone else.
Divrei Eliyahu: It hints that he fulfilled Sukah. Ba'Kol are the initial letters of "ba'Sukos Teshvu Shiv'as Yamim", "Kol ha'Ezrach b'Yisrael Yeshvu ba'Sukos", "Lema'an Yed'u Doroseichem..." (the three commands to dwell in the Sukah).
Da'as Zekenim: It alludes to why he was blessed - because he gave [to Shem] Ma'aser mi'Kol.
Oznayim la'Torah: Until now, the B'rachah of Hashem (at the beginning of Lech-Lecha) was not complete
See Ramban who explains the third opinion esoterically.
This was a blessing, because he would have had a problem - to whom to marry her off, to a Kena'ani, all of whom were cursed, or to a man from Charan, where they were all idolaters.
In fact, the Gemara there cites two more opinions: (1) That he was a master astrologer, at whose door all the kings of the east and west would assemble for counsel; (2) That a precious stone hung round his neck, which healed every sick person that saw it.
Mishnas R. Aharon 1 p.147: Even though he stood in 10 extremely hard tests, it was a Brachah that he was not tested further.
The Ramban says that ba'Kol hints to a Midah of Hashem called Kol. Chachmah u'Musar (4 p.9) says that this is Midas ha'Chesed - "Titen... Chesed l'Avraham." This explains how on the third day from his Bris, he was pained that there were no guests.
Why does the Torah need to tell us that Hashem had blessed Avraham... ?
Rashi: Now that he had a son, 1 he needed to find him a wife.
Seforno: Since he was wealthy, 2 he was afraid that someone inappropriate would bribe Eliezer into accepting his daughter as a Shiduch for Yitzchak. Which is why he went on to make him swear that he would follow his (Avraham's) instructions.
Ramban (citing the Ibn Ezea): Now that he had everything, all he wanted was grandchildren to inherit it all.