If we create a neural net that has the equivalent knowledge and intelligence of a human 5 year old but that neural net learns slower and was produced on 10 trillion pieces of data then we STILL have created a sentient AI despite the fact the learning algorithm is shit.
Which doesn't stop neural networks being immensely useful (and even significantly more useful than humans in relevant domains, especially five year old humans), but might well stop them from being sentient. Neural nets are probably already better at passing Turing tests than five year olds, but the five year old's love of mummy doesn't come from statistical analysis of a corpus of texts about love and mummies.
I would soften the idea a little bit by noting that as a little human grows up and learns more things, its brain plasticity goes down over time. AFAIK there’s no evidence that learning “saturates” the ability to learn other things, and the loss of plasticity is probably mainly linked to aging and/or other factors, but functionally the effect is very much the same: humans that know a lot of things typically have less capacity for learning.
https://www.youtube.com/watch?v=ZeiMhUlipTk
These people are clearly still sentient.
There is still some form of memory there but the overall mechanism is largely gone.