Megillah, Chapter Two, Mishnah Four
The first section of the mishnah deals with who is qualified to read the Megillah. The second section deals with when it can be read.
1) All are qualified to read the Megillah except a deaf person, an idiot and a minor.
a) Rabbi Judah qualifies a minor.
2) They do not read the Megillah, nor circumcise, nor go to the mikveh, nor sprinkling [purificatory waters], and similarly a woman keeping day for day should not take a ritual bath until the sun has risen.
a) But if any of these things is done after dawn, it is valid.
Section one: Deaf people, idiots (this refers either to one who is mentally retarded or an insane person) and minors are generally not obligated to perform the commandments. As we have mentioned on several occasions, in those days they had no way to communicate with the deaf and little understanding of the insane or retarded. Hence these people were mostly cut off from participating in society. Minors are too young to understand the meaning of the commandments and hence are not obligated. Since these people are not obligated, they cannot perform the commandment for others. This was stated also in Rosh Hashanah 3:8, at the end of that mishnah.
Rabbi Judah says that a minor is qualified. Assumedly he
Section two: The main reading of the Megillah takes place during the day. The Mishnah teaches that during the day means after the sun has risen. This is also the time for other mitzvoth that need to take place during the day. Sprinkling refers to the water mixed with the ashes of the red heifer. A woman keeping a day for a day refers to a woman who was impure do to non-menstrual genital discharge. She is impure a day for every day on which she has a discharge. All of these things should be done after the sun has risen. However, if someone did them after dawn, meaning when the sky begins to become light, the actions are still valid, at least ex post facto.