The Mishna (19b) teaches that anyone can read the Megilla for others, with the exception of a heresh, shoteh, ve-katan – a deaf-mute, an imbecile and a child. Rabbi Yehuda permits a katan to read the Megilla for others. While some of the commentaries try to explain how the case of the deaf-mute reading aloud for others is possible – suggesting that in this particular case we are talking about someone who is deaf and cannot hear, but can, in fact speak – the Talmud Yerushalmi simply says that we cannot possibly be talking about a heresh reading for others. The heresh is mentioned in the Mishna only because he is always partnered with the shoteh and the katan, but it has no true significance in our case.
With regard to the katan, the Gemara relates that Rabbi Yehuda not only permits a child to read the Megilla for others, in fact he testified that when he was a child, he read the Megilla in the presence of Rabbi Tarfon and the elders of Lod. His fellow Sages did not accept this as proof that a child can read for others, arguing that they do not accept proof from the story of a child. When Rabbi Yehuda HaNasi testified that as a child he read the Megilla for Rabbi Yehuda, his peers rejected this as any indication of normative practice, arguing that they do not accept proof from the activity of the person who himself was the one who permitted this behavior.
In truth, when dealing with issues of a Rabbinic nature – and keri’at Megilla certainly falls into that category – then we do accept the testimony of an adult who relates what he saw or experienced as a child. The Rid explains that keri’at Megilla is an exception, since it is a public reading from TaNaKH it is given the severity and significance of a Biblical law. The Birkei Yosef argues that this rule does not apply in our case, since we can only trust a childhood memory of a specific, clearly defined occurrence. In our case, Rabbi Yehuda merely reminisced about his reading the Megilla before Rabbi Tarfon and other Sages. That testimony does not clearly indicate that those Sages accepted the reading as valid – perhaps they listened to another rendition of the Megilla beforehand or afterwards.