The Torah relates that Moshe and Aharon came to Pharaoh. What happened to the elders?
Rashi: The elders slipped away (out of fear) one by one, so that, by the time Moshe and Aharon reached the palace, they had all disappeared. 1
That explains why, at Matan Torah, Hashem said that they were not to approach Har Sinai at all - Midah Keneged Midah (Rashi [See Sifshei Chachamim]).
Why did Hashem go to the trouble of warning Pharaoh? Why did He not simply destroy the Egyptians (for what they had done to Yisrael)?
Rashi (in Devarim): As Moshe explained (upon sending a message of peace to Sichon), he took his cue from Hashem, who could just as well have sent a bolt of lightning and wiped out the Egyptians; yet He sent him (Moshe) from the desert to ask Pharaoh - nicely to let His people go. 1
Seeing as Hashem planned to take Yisrael out of Egypt, why did he instruct Moshe to only ask for permission for Yisrael to sacrifice in the desert?
Refer to 3:18:2:1,2.
Because the earliest time for the termination of the Galus would only fall due in the following year. Meanwhile, He wanted Pharaoh to acknowledge Hashem's Omnipotence, and that Yisrael were His people.
Rashi writes that the elders slipped away, and therefore at Matan Torah, they were not allowed to come close. Aharon came with Moshe, but also he could not come as close as Moshe!
Riva, Moshav Zekenim: Moshe is mentioned before Aharon, for Aharon did not invigorate himself for the Mitzvah [to come to Pharaoh] as much as Moshe did.