Slower development usually means a lower plateau, and I think we pretty much have to assume as such (and can be prepared to be pleasantly surprised).
Else, we get to wishful thinking: older people on older devices developed more slowly and plateaued at a lesser value of hearing. Now, we have implanted older people on newer devices, and they're developing more slowly, but hey, maybe they'll eventually develop fully normal hearing.
But if you want cochlear implant specific data, here-- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10760633/
You're right that time narrows the gap between early implantation and later implantation, but the slope of that narrowing is pretty small by the 20 year mark (and barely statistically significant in this moderately-sized study) and the gap is relatively big.
The difference of time of implantation between the two groups was relatively small (mean implantation at 45 months vs. 34 months) and produces a gap that's durable for decades. >130 months is way, way, out from 45 months.
Still, lots of work to do, to quote "However, it was not possible to control other factors, such as the socio-economic environment of the participants.".
In my view this could affect the study quite a lot (or not, but unknown for now). They mention the initial intervention was 3 months on-site, but after "the patient returned to his area of residence, where he/she would have speech therapy and special education".
However, slower adaptation isn’t zero adaptation. The limits for people implanted at 5 when they are 50 is still an open and IMO interesting question.