Muggles' Guide to Harry Potter/Magic/Underage Sorcery
|Muggles' Guide to Harry Potter - Magic|
|Decree for the Reasonable Restriction of Underage Sorcery|
|Type||Legal structure (Law)|
|Features||Prevents underage wizards from using magic|
|First Appearance||Harry Potter and the Philosopher's Stone (mentioned), Harry Potter and the Chamber of Secrets (named)|
The Decree for the Reasonable Restriction of Underage Sorcery is a law that limits the magic that underage wizards are allowed to use. The apparent intent is to keep young, untutored wizards from running amok with their wands and causing consternation among the Muggles.
Extended Description 
Our first introduction to this statute is the note given to every student at the close of each school year; Harry first receives his at the end of Harry Potter and the Philosopher's Stone. Fred and George comment they always hope the note will be forgotten one year, but it never is. While not mentioning the rule by name, the note informs students they are forbidden to perform magic outside of school until they come of age.
Harry first runs afoul of this law in Harry Potter and the Chamber of Secrets when Dobby uses a Hover Charm to destroy a dessert in the Dursleys' kitchen. Evidently, the Ministry assumes Harry performed the magic, because a letter is sent by one Mafalda Hopkirk warning him not do that again.
When Harry accidentally inflates his Aunt Marge, he is deathly afraid that, having violated this Statute, he may now be expelled from the Wizarding world. He decides his only recourse is to become a fugitive, but as he is approaching Diagon Alley to retrieve his money, he is intercepted by Cornelius Fudge, the Minister for Magic. Fudge inexplicably dismisses the inflating incident.
When Harry uses the Patronus charm to protect Dudley from Dementors, he is once again told that he has violated this Decree, and his wand will be destroyed. This results, eventually, in a hearing, at which it is determined that Harry was justified in using magic to defend himself and a Muggle bystander against a magical threat.
Finally, Harry is overjoyed when, on his seventeenth birthday, he is free to use any magic he chooses without hindrance.
In Harry Potter and the Deathly Hallows, we learn that the technique that is used to determine whether a Wizarding child has performed magic is called the Trace. This apparently will detect a child using magic by triggering some alert within the Ministry. Apparently, in order to be sensitive enough to register the child using small spells, it also detects other magic being used nearby. We are told that in Wizarding households, it is the parents responsibility to monitor their child using magic, as adults using magic near him may trigger the Trace accidentally; Muggle-born wizards, however, are directly monitored by the Ministry, as it is assumed that the only magic that can occur near them is their own. This is presumably why Dobby's Hover charm was attributed to Harry.
One must wonder, though, about the Trace's accuracy. There is no apparent notice when Aunt Marge's glass magically shatters, or the cabinet under the stairs magically unlocks as Harry leaves Privet Drive. There is no apparent notice when magic occurs in Harry's vicinity when he stays at the Burrow, Grimmauld Place, or in Diagon Alley. There is no notice of the magic occurring in his vicinity when he is being visited by Arthur Weasley, the Advance Guard, or Professor Dumbledore at Privet Drive, or when Hagrid is taking him off the island and back to shore in the first book. If we assume that the Trace can detect an adult wizard, and damp its responses accordingly, then the only unexplained situations are the shattering wineglass and the opening cupboard during Aunt Marge's stay at Privet Drive, and possibly the summoning of the Knight Bus. Those, along with Aunt Marge's inflation, might have been lumped together and forgiven by Cornelius Fudge.
However, it is also true that the Ministry does not seem to be aware of Harry's use of magic before he turns eleven. In the first chapters of Harry Potter and the Philosopher's Stone, Harry unknowingly shrinks a horrible sweater down to doll-size so that he won't have to wear it, flies to a rooftop to avoid a beating from Dudley and his gang, regrows his hair overnight, and makes the glass vanish from a snake's cage. However, there are no warnings issued by the Ministry during this time. Possibly the Trace is only activated when the child reaches the age of 11 and is assumed to be consciously able to direct his or her magic. Could the law also prevent selling or giving wands to children under the age of 11?
This still leaves issues with the Trace remaining quiescent when the Twins and Ron are in Arthur's flying car, when Harry joins them, and when Harry and Ron are flying the car to Hogwarts. Certainly the car is a magical artifact, and so the Trace should be active, but none of the underage wizards involved in that escapade receive warnings. Another writer, in another series (Randall Garrett, in the Lord Darcy series), has drawn a distinction between active and passive magic, saying that possessing or using a charmed object is significantly different than actively creating a new spell. It is probably necessary that this same distinction be built into the Trace, as the number of charmed objects in the ordinary Wizarding household that would be used by a child is quite extensive — an infant banging on a self-stirring cauldron with a spoon, for instance, could be considered someone using a magical object. If this distinction was not made by the Trace itself, the Ministry would be overwhelmed with reports of magical objects being used by underage wizards.
- How old do Wizarding children have to be in order to legally use magic outside of school?