Rashi, the Midrash Rabbah, and many others mention the story of Avraham being thrown into a fiery furnace by Nimrod for refusing to worship Avoda Zara and being miraculously saved. I've spent a while looking but I haven't managed to find any early sources discussing how old he was at the time. I've always heard he was 48 but I haven't found any early sources corroborating this (I did find a few blogs or similar websites saying different ages including 48, but none of them quote any source). Can anyone find any sources (I'd prefer Rishonim or earlier, but its not necessary)? Thank you so much.
Rambam (Hilchos Avodah Zarah 1:3) seems to say 40. The below translation is from Sefaria, emphasis mine.
כֵּיוָן שֶׁנִּגְמַל אֵיתָן זֶה הִתְחִיל לְשׁוֹטֵט בְּדַעְתּוֹ וְהוּא קָטָן וְהִתְחִיל לַחֲשֹׁב בַּיּוֹם וּבַלַּיְלָה וְהָיָה תָּמֵהַּ הֵיאַךְ אֶפְשָׁר שֶׁיִּהְיֶה הַגַּלְגַּל הַזֶּה נוֹהֵג תָּמִיד וְלֹא יִהְיֶה לוֹ מַנְהִיג וּמִי יְסַבֵּב אוֹתוֹ. כִּי אִי אֶפְשָׁר שֶׁיְּסַבֵּב אֶת עַצְמוֹ. וְלֹא הָיָה לוֹ מְלַמֵּד וְלֹא מוֹדִיעַ דָּבָר אֶלָּא מֻשְׁקָע בְּאוּר כַּשְׂדִּים בֵּין עוֹבְדֵי כּוֹכָבִים הַטִּפְּשִׁים וְאָבִיו וְאִמּוֹ וְכָל הָעָם עוֹבְדֵי כּוֹכָבִים וְהוּא עוֹבֵד עִמָּהֶם וְלִבּוֹ מְשׁוֹטֵט וּמֵבִין עַד שֶׁהִשִּׂיג דֶּרֶךְ הָאֱמֶת וְהֵבִין קַו הַצֶּדֶק מִתְּבוּנָתוֹ הַנְּכוֹנָה. וְיָדַע שֶׁיֵּשׁ שָׁם אֱלוֹהַּ אֶחָד וְהוּא מַנְהִיג הַגַּלְגַּל וְהוּא בָּרָא הַכּל וְאֵין בְּכָל הַנִּמְצָא אֱלוֹהַּ חוּץ מִמֶּנּוּ. וְיָדַע שֶׁכָּל הָעוֹלָם טוֹעִים וְדָבָר שֶׁגָּרַם לָהֶם לִטְעוֹת זֶה שֶׁעוֹבְדִים אֶת הַכּוֹכָבִים וְאֶת הַצּוּרוֹת עַד שֶׁאָבַד הָאֱמֶת מִדַּעְתָּם. וּבֶן אַרְבָּעִים שָׁנָה הִכִּיר אַבְרָהָם אֶת בּוֹרְאוֹ. כֵּיוָן שֶׁהִכִּיר וְיָדַע הִתְחִיל לְהָשִׁיב תְּשׁוּבוֹת עַל בְּנֵי אוּר כַּשְׂדִּים וְלַעֲרֹךְ דִּין עִמָּהֶם וְלוֹמַר שֶׁאֵין זוֹ דֶּרֶךְ הָאֱמֶת שֶׁאַתֶּם הוֹלְכִים בָּהּ וְשִׁבֵּר הַצְּלָמִים וְהִתְחִיל לְהוֹדִיעַ לָעָם שֶׁאֵין רָאוּי לַעֲבֹד אֶלָּא לֶאֱלוֹהַּ הָעוֹלָם וְלוֹ רָאוּי לְהִשְׁתַּחֲווֹת וּלְהַקְרִיב וּלְנַסֵּךְ כְּדֵי שֶׁיַּכִּירוּהוּ כָּל הַבְּרוּאִים הַבָּאִים. וְרָאוּי לְאַבֵּד וּלְשַׁבֵּר כָּל הַצּוּרוֹת כְּדֵי שֶׁלֹּא יִטְעוּ בָּהֶן כָּל הָעָם כְּמוֹ אֵלּוּ שֶׁהֵם מְדַמִּים שֶׁאֵין שָׁם אֱלוֹהַּ אֶלָּא אֵלּוּ. כֵּיוָן שֶׁגָּבַר עֲלֵיהֶם בִּרְאָיוֹתָיו בִּקֵּשׁ הַמֶּלֶךְ לְהָרְגוֹ וְנַעֲשָׂה לוֹ נֵס וְיָצָא לְחָרָן. וְהִתְחִיל לַעֲמֹד וְלִקְרֹא בְּקוֹל גָּדוֹל לְכָל הָעוֹלָם וּלְהוֹדִיעָם שֶׁיֵּשׁ שָׁם אֱלוֹהַּ אֶחָד לְכָל הָעוֹלָם וְלוֹ רָאוּי לַעֲבֹד. וְהָיָה מְהַלֵּךְ וְקוֹרֵא וּמְקַבֵּץ הָעָם מֵעִיר לְעִיר וּמִמַּמְלָכָה לְמַמְלָכָה עַד שֶׁהִגִּיעַ לְאֶרֶץ כְּנַעַן וְהוּא קוֹרֵא שֶׁנֶּאֱמַר (בראשית כא לג) "וַיִּקְרָא שָׁם בְּשֵׁם ה' אֵל עוֹלָם". וְכֵיוָן שֶׁהָיוּ הָעָם מִתְקַבְּצִין אֵלָיו וְשׁוֹאֲלִין לוֹ עַל דְּבָרָיו הָיָה מוֹדִיעַ לְכָל אֶחָד וְאֶחָד כְּפִי דַּעְתּוֹ עַד שֶׁיַּחְזִירֵהוּ לְדֶרֶךְ הָאֱמֶת עַד שֶׁנִּתְקַבְּצוּ אֵלָיו אֲלָפִים וּרְבָבוֹת וְהֵם אַנְשֵׁי בֵּית אַבְרָהָם וְשָׁתַל בְּלִבָּם הָעִקָּר הַגָּדוֹל הַזֶּה וְחִבֵּר בּוֹ סְפָרִים וְהוֹדִיעוֹ לְיִצְחָק בְּנוֹ. ...
As soon as this giant was weaned he commenced to busy his mind, in his infancy he commenced to think by day and by night, and would encounter this enigma: How is it possible that this planet should continuously be in motion and have no leader—and who, indeed, causes it to revolve, it being impossible that it should revolve itself? Moreover, he neither had a teacher nor one to impart aught to him, for he was sunk in Ur of the Chaldeans among the foolish worshipers of stars, and his father, and his mother, like all the people, worshiped stars, and he, although following them in their worship, busies his heart and reflects until he attains the path of truth, and, by his correct thinking, he understood when he finally saw the line of righteousness. He knew that there is One God; He leads the planet; He created everything; and in all that is there is no god save He. He knew that the whole world was in error, and that the thing which caused them to err was, that their worshiping the stars and the images brought about the loss of the truth from their consciousness. And, when Abraham was forty years old he recognized his Creator. After he came to this comprehension and knowledge he started to confute the sons of Ur of the Chaldeans, and to organize disputations with them, cautioning them, saying: "This is not the true path that you are following", and he destroyed the images, and commenced preaching to the people warning them that it is not right to worship any save the God of the universe, and unto Him alone it is right to bow down, to offer sacrifices, and compound offerings, so that the creatures of the future shall recognize Him. Moreover, it is right to destroy and break in pieces all of the images, so that the whole population of the future be not led to an error like unto these who imagine that there is no God save these images. When he had them subdued by his well supported contentions, the king tried to put him to death, but he was saved by a miracle, and went hence to Haran. There he stood up anew and called out in a great voice to the whole world, to let them know that there is One God for the whole universe, and unto Him it is proper to render service. And thus he went onward with his proclamations from city to city, and from government to government, until he attained the land of Canaan amidst his outcry, even as it is said: "And called there on the name of the Lord, the Everlasting God" (Gen. 21.33). When the people who congregated about him asked him concerning his preachments, he replied by imparting knowledge to each and every one according to his mentality, to the end that he was able to turn him to the path of truth, until there congregated about him thousands, even tens of thousands, and they became the people of Abraham's household, in whose heart he implanted this great cause, concerning which he compiled books, and which he imparted to his son Isaac. ...
While the Rambam doesn’t explicitly discuss the Kivshan HaEish, it is implied that this is what he is referring to.
It should be noted that while the Rambam holds that Avraham began his journey to discovering Hashem at 3 and ultimately began preaching monotheism at 40, the Raavad argues and places the entire account at 3.
ובן ארבעים שנה הכיר אברהם את בוראו. א''א יש אגדה בן שלש שנים שנאמר עקב אשר שמע אברהם בקולי מנין עק''ב:
“And, when Abraham was forty years old he recognized his Creator.” Says the Raavad: There is an Aggadah [that Abraham was] three, as it says, “Since [עקב] Abraham listened to my voice,” from the numerical value of עקב [i.e. 172, taken from his age at death, 175, yields 3].
The Kesef Mishnah notes that it is a debate when Abraham recognized Hashem (Nedarim 32a), whether at 3 or 48. He posits that the Rambam had the reading of “40” for that latter opinion, and he combines the two opinions, as above. However, it would come out that according to our reading of the Gemara, that he recognized Hashem at 48, the story of the Kivshan HaEish took place when Avraham was 48.
Finally, one of the few Midrashim that lists Avraham’s ten tests actually gives us a benchmark for his age. In Pirkei d’Rebbi Eliezer 26, we are told:
עשרה נסיונות נתנסה אברהם אבינו, ועמד בכולן. הנס הראשון, כשנולד אברהם בקשו כל גדולי מלכות והקוסמים להרגו ונחבא בבית הרן שלש עשרה שנה ולא ראה השמש ולא הירח ולאחר שלש עשרה שנה יצא מבית הרן מדבר בלשון הקדש ומאס באלילים ומשקץ את הפסילים ובטח בשם יוצרו ואמר אשרי אדם בוטח בך.
הנס השני, נחבש בבית האסורים עשרה שנים שלשה בכותי ושבעה בבי דרי ולאחר עשר שנים שלחו והביאו אותו אצלו והשליכוהו לתוך כבשן האש ומלך הכבוד פשט יד ימינו והצילו מכבשן האש שנאמר ויאמר אליו אני ה' אשר הוצאתיך מאור כשדים.
With ten tests was our forefather Abraham tested, and he stood in all of them. The first test: when Abraham was born, all of the kings and magicians wished to kill him, and he was hidden underground for thirteen years, without seeing the sun or the moon. After thirteen years, he emerged, speaking the Holy Language, despising idols and being disgusted by images. He trusted in Hashem His creator, and he said, “Praiseworthy is the man who trusts in You.”
The second test: He was imprisoned for ten years, three in Kusi and seven in Bei Dari. After ten years, they sent and brought him next to him, and they threw him into a fiery furnace, and the King of Glory stretched out His right hand and saved him from the fiery furnace, as it says, “And He said to him, ‘I am Hashem Who took you out from the furnace of the Chaldeans.’”
So for the first thirteen years of his life, Avraham was in hiding, and for the ten years concluding with the Kivshan HaEish, he was imprisoned. As we’re not told the time gap between these events, we can’t be sure of an exact age, but this does tell us that at a minimum, Avraham was 23 when he was thrown into the pit.
In summary: According to the Rambam’s reading of Nedarim, Avraham was 40. According to our reading of Nedarim, Avraham was 48. According to the Raavad, Avraham was 3. According to Pirkei d’Rebbi Eliezer, Avraham was at least 23.