It was my understanding that the halakha differentiates between a person who cannot hear and one who can neither speak nor hear, despite the fact that the Hebrew word חרש is used for both (cf, for example: Terumot 1:1-2). It was also my understanding that a person who can neither speak nor hear is one who was born deaf, while a person who cannot hear but who can speak was one who became deaf later in life. In fact, this is how Kehati appears to understand the term in Terumot 1:2 - "חרש המדבר ואינו שומע - כגון שהיה שומע כשנולד ואחר כך איבד את כושר השמיעה" ("A deaf person who can speak but who cannot hear - such as one who could hear when he was born, but who subsequently lost the faculty of hearing").
Bearing that distinction in mind, can somebody please explain to me how it is that Yevamot 14:1 can speak of a person "becoming" deaf, and subsequently becoming incapable of effecting a divorce? Given that the mishna's interpretation is dependant upon them being non compos mentis, I would think that it needs be referring to the case of a person who can neither speak nor hear, which is somebody born deaf.
I cannot think of how a person might suddenly be stricken both deaf and dumb later on in life, such that they are no longer to be considered capable of effecting a divorce. [- or somebody who suffered a certain type of brain damage; see comment beneath this question.]
More particularly, I am asking the following: is the person spoken of in Yevamot 14:1 a person who was stricken both deaf and dumb later in life, or is he simply a person who has lost his hearing? Is a person who has lost his/her hearing to be considered non compos mentis as a result?