What exactly is the Torah telling us here?
Seforno: That Moshe carried out the procedure of each Shelichus exactly according to Hashem's instructions. 1
And why does it add "Kein Asu"?
Seforno: To teach us that they neither added anything nor subtracted anything from Hashem's words.
Why does the Torah see fit to inform us the ages of Moshe and Aharon?
Rashi (in Sotah, 12a): Because it was in the same year that Yisrael left Egypt.
Seforno: Bearing in mind that, already then, eighty was considered passed the stage of 'old age (Seivah)' and into that of 'Gevuros', as the Pasuk in Tehilim, 90:10 - composed by Moshe - informs us.
See Oznayim la'Torah who learns from here the futility of retirement to relax and enjoy one's final years in quietude.