But once they learn to read and write shouldn’t they be indisinguisable from a non-deaf writer?
The only profoundly deaf people I’ve met who could read and write English that well were either later-deafened or had some residual hearing when they were young. (Again—I sound like a broken record—I haven’t had as much contact with the people these days getting cochlear implants at a young age.)
It sounds like you’re saying it’s harder to learn, but what you said earlier seemed to indicate it was harder for them to understand after having already learned.
No—you’re right on this. It does seem like language acquisition by a critical age is the biggest thing.
One of my last clients when I was interpreter fits this. She lost her hearing due to an illness at something like 8 or 10 years of age. When I knew her, she was absolutely completely deaf. She was a wizard with speech reading (though that has all sorts of limitations—line of sight, people who cover their mouths frequently, shaggy facial hair, foreign accents, lighting, etc.), and her English reading and writing was indistinguishable from a hearing person’s. In fact, her academic skills in general were well above average. She also had a good concept of sound and music—even though she hadn’t heard any in years. She could even speak pretty well. Her biggest problem was no clue of appropriate volume for a room—she’d be way too quiet in noisy rooms, for instance.
She eventually got a cochlear implant as an adult, and with her English already in place (and, I think, her brain already organized for sound), she basically became a hearing person nearly overnight. It was dramatic.
Other adults who’ve had the surgery consider it the worst mistake they’ve ever made—all they got were unpleasant noise sensations. Others only hoped to get some idea of environmental sounds (like sounds of cars for safety purposes).

