לֵ֣ךְ וְאָֽסַפְתָּ֞ אֶת־זִקְנֵ֣י יִשְׂרָאֵ֗ל וְאָמַרְתָּ֤ אֲלֵהֶם֙ יְהוָ֞ה אֱלֹהֵ֤י אֲבֹֽתֵיכֶם֙ נִרְאָ֣ה אֵלַ֔י אֱלֹהֵ֧י אַבְרָהָ֛ם יִצְחָ֥ק וְיַעֲקֹ֖ב לֵאמֹ֑ר פָּקֹ֤ד פָּקַ֙דְתִּי֙ אֶתְכֶ֔ם וְאֶת־הֶעָשׂ֥וּי לָכֶ֖ם בְּמִצְרָֽיִם׃
Go, and gather the elders of Israel together, and say unto them: The LORD, the God of your fathers, the God of Abraham, of Isaac, and of Jacob, hath appeared unto me, saying: I have surely remembered you, and seen that which is done to you in Egypt.
Rashi on Exodus
את זקני ישראל THE ELDERS OF ISRAEL, those singled out to attend meetings, (i. e. the leaders) (cf. Yoma 28b). If, however, you should say that the text means old men in the ordinary sense I ask: How was it possible for him to gather all the old men amongst a male population of 600,000?
Or HaChaim on Exodus
לך ואספת את זקני ישראל, "Go and gather the elders of lsrael,etc." The reason that G'd repeats here again what Moses is to say maybe that when Moses would arrive in Egypt he was to tell the entire people that he had come as messenger from G'd. After that he would assemble the elders and tell them the nature of his mission, namely that G'd had remembered His people and was about to redeem them.