Rashi's explanation of why we are told that Avram was 86 at the birth of Yishma'el

Question

Bereishit 16:16:

וְאַבְרָ֕ם בֶּן־שְׁמֹנִ֥ים שָׁנָ֖ה וְשֵׁ֣שׁ שָׁנִ֑ים בְּלֶֽדֶת־הָגָ֥ר אֶת־יִשְׁמָעֵ֖אל לְאַבְרָֽם׃‏

Avram was eighty-six years old when Hagar bore Yishma'el to Avram.

Rashi comments:

לְשִׁבְחוֹ שֶׁל יִשְׁמָעֵאל נִכְתַּב, לְהוֹדִיעַ שֶׁהָיָה בֶּן י"ג שָׁנָה כְּשֶׁנִּמּוֹל וְלֹא עִכֵּב:‏

This is written to Yishma'el’s credit, pointing out that he was thirteen years old when he was circumcised and yet he raised no objection.

I don't understand what Rashi is saying here. We have an explicit verse (Bereishit 17:25) that tells us that Yishma'el was thirteen years old when he was circumcised.

What is Rashi inferring from Bereishit 16:16 which we would not know from Bereishit 17:25?

Answer 1

R. Judah Loewe in his supercommentary there explains that the explicit verse would only tell us the fact. The implication of 16:16 is to highlight the praiseworthiness of this fact:

אף על גב דכבר כתב בסוף הפרשה בפירוש שהיה בן י”ג כשנמול מכל מקום רצה לומר שזה הכתוב מדבר בשבחו דאילו ממקרא דלקמן לא הוי ידעינן שהכתוב מדבר בשבחו ולכך נכתב זה הפסוק לומר שהוא מדבר בשבחו

Even though it already wrote explicitly at the end of the parshah that he was 13 years old when he was circumcised, nevertheless he means to say that this verse is speaking of his praise. For if [the only source was] from the verse later on we wouldn’t know that it is speaking of his praise. Therefore it wrote this verse to say that it is speaking of his praise.

Answer 2

The Riva says that from the second verse, you would only know the age, but from 16:16, you can see that it was not done against Yishma'el's will.

חשיב דמהתם נוכל לומר דבעל כרחו נמול לכך למד כאן רש"י מיתורא שמרצונו היה.

While it doesn't say so, this might be developed from the last word, "l'Avram" that he was born not just to Hagar, but to Avram and he emulated Avram's willingness to be circumcised.

The Siftei Chachamim writes

You might ask: [If the reason is as Rashi says,] why is it expressly written, “And his son Yishmael was 13 years old when he circumcised...” (17:25)? The answer is: Scripture repeats it to emphasize that Yishmael circumcised himself because of Hashem’s mitzvah, not from fear of his father who obliged him to do it. But the Mahara’i answers: We would not know from v. 17:25 that Yishmael was a full 13 years old, which is the age of attaining maturity, [for it could mean he was just over 12, and thus still a minor,] and that is why he did not resist. But the verse here makes the point clear.