Late-bloomer

A Late-bloomer was a wizard or witch whose magicel powers did not manifest until their adulthood. This is a very event, and the magic finally revealed itself when the witch or wizard was in a very desperate situation. These individuals were able to attend Hogwarts School as adults.

Typically, a wizard's powers manifest between ages 7 and 11.

Behind the Scenes

 * JKR had planed to include a late-bloomer in early drafts of her books, but changed her mind while writing Harry Potter and the Prisoner of Azkaban. It is not entirely if this makes the existence of late-bloomers non-canon, though.
 * There was some speculation that Petunia Dursley was a late-bloomer witch. JKR has made clear that Petunia is a Muggle.