To attend Hogwarts School of Witchcraft and Wizardry, students must be at least 11 years old. This is the age at which they receive their letter of acceptance to the school, and are able to begin their studies.
When a student was Muggle-born the acceptance letter was delivered in person by a member of the Hogwarts faculty. The faculty member acted as a special messenger, who explained everything to the student and parents.
Harry Potter first received one during the summer holidays of 1991.
On Harry Potter's 11th birthday he begins to receive letters inviting him to enroll at the Hogwarts School of Witchcraft and Wizardry. Each is addressed in green ink to “Mr H. Potter, the Cupboard under the Stairs, 4, Privet Drive, Little Whinging, Surrey.”
On his eleventh birthday, Harry discovers he is a wizard when Rubeus Hagrid brings him an acceptance letter from Hogwarts School of Witchcraft and Wizardry. Harry learns that his parents, James and Lily Potter, were murdered by a powerful Dark wizard, Lord Voldemort, who also tried to kill Harry.
There is only one way that a student is selected to go to Hogwarts, and that choice isn't made by a person. In fact, it is made by a book and quill. In a tower at Hogwarts, where no student goes, sits the Book of Admittance and Quill of Acceptance. These objects decide who gets a place at Hogwarts.
Each year, the Registry produces a shortlist of magical children who are approaching age 11. These children receive acceptance letters by owl or muggle post. It is possible, during the updating and transfer of the Registry, that names have been omitted from recent lists.
The letter is written on parchment paper and delivered to the recipient by owl, except in the case of Muggle-borns and magical children living with Muggles (such as Harry Potter), where the initial acceptance letter is delivered by the regular Muggle post.
Professor McGonagall will ask you what the returning students will find in their Hogwarts letters. The correct answer is “School supply lists”. She will point out that both new and returning students need to be properly equipped for the lessons to come.
For the actual prop, they didn't use a font, as it was handwritten by Miraphora Mina (one of the main graphic designers on all the Harry Potter – and now Fantastic Beasts – films). The font you see here is a recreation which made by a fan (you can read about that here: The Perfect Hogwarts Acceptance Letter…)