He wrote the suicide letter when he was 31, saying he's been suffering for 6 years. Are you saying he lied about his hearing in a suicide letter, or that late 20's is a regular age to get sensineural hearing loss?
people can get sensineural hearing loss at any age, from a multitude of causes. backing up, there are two main categories of hearing loss, sensineural and conductive. conductive refers to problems with the ear canal, ear drum, middle ear space, or ossicles (the small ear bones, aka hammer/anvil/stirrup, aka malleus, incus, stapes). sensineural refers to problems with the inner ear (cochlea) or the coclear nerve (eighth cranial nerve).
the hearing loss that people get due to aging is called presbycusis, and is a type of sensineural loss. most people don't get significant presbycusis until at least 50s-60s, but people who have been exposed to loud noises can certainly start to show decline in their 20s-30s (although again, that is early).
the cause of beethoven's hearing loss is still something of a mystery. the book I referenced concluded that it was plain old presbycusis, but there's also a few unexplained features, such as the fluctuating nature of the loss, which is really only seen in meniere's disease, which there is no evidence of him having. labyrinthitis is unlikely due to the progressive nature and the lack of vertigo. autoimmune causes don't really make sense because those are usually much more rapid in onset, and he didn't have any other autoimmune diseases. otosclerosis certainly could explain his problem, but without a premortem ear exploration surgery or pathologic exam of his temporal bones, you'd never be able to prove that.
there's also a few people who think that he was not actually deaf, but that his deafness was a conversion disorder (ie all in his head, and not real). this is different than "faking it," in that he would honestly have believed that he was deaf, even though he actually wasn't. the only way to prove that would be a few specialized hearing tests (not always done on a routine audiogram).
what I can say with certainty is that beethoven never had a modern audiogram, and without that, it's pretty hard to say what exactly was going on with his hearing.
back to the real topic though, opus 111 is indeed a great sonata. I prefer the middle sonatas more myself, but that's just my taste.