The L-rd ... gave them a charge to the Children of Israel (Ex. 6:13)
Despite the fact that the Jewish people hadn't listened "because of their anguished spirit and the cruel slavery," G-d commanded Moses and Aaron to keep on talking. For the word of G-d always makes an impression and has an effect: if not immediately, then sometime later. Holy words are never wasted, and are always ultimately heard. (Sefat Emet)
And Moses was eighty years old, and Aaron eighty-three years old, when they spoke to Pharaoh (Ex. 7:7)
Why does the Torah need to tell us the ages of Moses and Aaron? To refute the common misconception that only young people can carry the banner of liberation and redemption. Older people, too, can be "revolutionaries," if G-d determines it is necessary and the proper time. (Shaarei Yerushalayim)
And the frogs died in the houses, in the villages, and in the fields (Ex. 8:9)
When the plague of frogs was over, the frogs died. By contrast, after the plague of "various wild beasts" the animals did not die, but went back to wherever they had come from. The reason is that no "new" animals were created for the plague of "various wild beasts"; at G-d's command they left their natural habitat and converged on Egypt. When the plague ended, they were still necessary for the world's ecosystem. The frogs, however, were created especially for the plague; when it was over, there was no need for so many. (Kehilat Yitzchak)