וַתִּפְתַּח֙ וַתִּרְאֵ֣הוּ אֶת־הַיֶּ֔לֶד וְהִנֵּה־נַ֖עַר בֹּכֶ֑ה וַתַּחְמֹ֣ל עָלָ֔יו וַתֹּ֕אמֶר מִיַּלְדֵ֥י הָֽעִבְרִ֖ים זֶֽה׃
When she opened it, she saw that it was a child, a boy crying. She took pity on it and said, “This must be a Hebrew child.”
והנה נער בכה. קוֹלוֹ כְּנַעַר :
והנה נער בכה lit., AND BEHOLD A BOY WEEPING — Although he was a ילד, “a child”, his voice was like that of a נער, a grown up boy.
Rashi on the Gemara in Sotah which makes the same point makes it very clear that it is an older boy:
נער - גדול משמע:
What on Earth is the point of Moshe's voice sounding like a grown up boy? There must be some very serious significance which necessitated this miracle but I don't have the faintest idea what it could be.
(It would have been very clear to everyone that he wasn't actually an older boy since there is no indication that he looked like an older boy so this could not possibly have been to trick Bas Pharaoh into thinking he was born before the decree.)