9Age at implantation & outcomes
This module is where the biology of the whole chapter becomes a number on a clinic chart. If plasticity declines with age, and if the implant works by supplying activity while the brain is still plastic, then the age at which a deaf child is implanted should predict how well they do — and it does, more powerfully than almost any other single factor. But the rule is not universal: the same chart that makes timing decisive for one child barely applies to another, and understanding why is the heart of candidacy counselling.
FTThe strongest single factor
Across study after study, in a child deaf from birth, the age at implantation is among the strongest predictors of eventual speech and language — stronger, often, than the brand of device or the details of the surgery. Earlier is better, and the relationship is steep: the difference between implanting at one year and at five is not a small increment but a different developmental trajectory. This is the sensitive period of the previous modules, expressed as outcome.[2002, 2012]
The clinical data bear this out. In the large CDaCI cohort, the younger the child at implantation, the faster their subsequent spoken-language growth. And the same urgency applies one step earlier, at diagnosis: children whose hearing loss is identified by about six months of age — and given prompt intervention — develop language far closer to normal than those identified later, whatever the degree of loss.[2010, 1998]
TCTwo populations, two rules
The crucial subtlety is that “age at implantation” means different things for different recipients. For the prelingually deaf child — deaf before language was acquired — age at implantation tracks how much of the sensitive period remains, and outcome falls sharply with delay. For the postlingually deaf adult — who lost hearing after language was already established — the linguistic brain is largely built and preserved, so age at implantation per se matters far less. Switch between the two below.[2010]
This is why a 60-year-old who lost hearing in adulthood can do beautifully with an implant placed decades after “the window,” while a 6-year-old deaf from birth, implanted at the same chronological remove from their deafness onset, may struggle. The clock that matters is not age in the abstract but age relative to when the auditory and language systems were built.
TCDuration of deafness & residual hearing
Two further factors refine the picture. The duration of deafness — how long the pathway was deprived before input was restored — is its own prognostic variable, and even in postlingual adults a very long deafness erodes the result, because the deprivation changes of earlier modules accumulate. And any residual hearing — even minimal, even aided — tends to predict better outcomes, because the auditory pathway was never wholly silent: some activity, however impoverished, kept the system partly engaged through the window.[2009]
FTThe clinical bottom line
For the family of a deaf infant, the message is urgency: identify the loss early (newborn screening) and implant within the sensitive period, because every month of delay within it costs achievable outcome. For the adult who has lost hearing, the message is reassurance: the window that governs the deaf child does not bar you, and even a long-standing loss is worth treating — though sooner is still better. The same plasticity science yields both.
We have now traced plasticity from the developing brain to the implanted child. The remaining modules turn to the parts of the story that are less about the young brain — how the adult brain still changes, how the two ears together develop, and what the limits of plasticity finally mean: back to the implant as input or on to adult plasticity.
Who is likely to derive more open-set speech benefit from implantation, and why?
In a child deaf from birth, how important is age at implantation relative to other factors?
Why does age at implantation matter far less for a postlingually deaf adult than for a prelingually deaf child?
Why does even minimal residual hearing tend to predict a better implant outcome?