Why does the Torah mention Moshe before Aharon (in Pasuk 13 and Pasuk 7), wwhereas here it mentions Aharon first?
Rashi: To teach us that Moshe and Aharon were on a par with one another (in righteousness). 1
Rashbam and Hadar Zekenim (in Pasuk 20): Regarding the Shelichus, it places Moshe first (since he was the chief Shali'ach, whereas regarding the Yichus, it places Aharon first (since he was older than Moshe).
Oznayim la'Torah #1: In reality, Moshe was the Ikar, and Hashem only appointed Aharon to accompany Moshe because Moshe initially refused. And it is to demonstrate that did not play a secondary role in the Sh;lichus that the Torah mentions him here before Moshe. 2
Oznayim la'Torah #2: When the Torah writes that Moshe and Aharon were equal, it means that, although Moshe was three years Aharon's junior, he had achieved in his eighty years what Aharon had achieved in his eighty-three years, 3 which means that Moshe was greater than Aharon. 4
The Tosefta at the end of Kerisos states that this is the only place where the Torah mentions Aharon before Moshe. Everywhere else it mentions Moshe first.
See Oznayim la'Torah, who elaborates further.
Oznayim la'Torah: Which explains why the Torah presents Moshe and Aharon's ages later in 7:7.
Why does the Torah see fit to insert "Hu Aharon u'Moshe ... "?
Rashi: The Torah is merely clarifying the identity of the current two leaders.
Seforno: It is pointing out that Moshe and Aharon were the most prestigious of their family, and were worthy of taking Yisrael out of Egypt, since their words would be accepted by all of Yisrael.
What are the implications of "Al Tziv'osam"?