This is a fundamental misunderstanding of what it means to grow up. There are certain skills, like learning to speak, that can only be done at a certain age. Once that age is past it is not possible to ever pick them up and be fluent at them.

Covid for under 12s was a disaster. We might as well have fed those children lead for the mental retardation we've caused them for the rest of their lives. Over a virus that killed precisely none of them.