“What ages is Taking Children Seriously most powerful for, or how does its value change with age? When is it almost too late?”
There is no age warranting not taking the person seriously. We should take everyone seriously, irrespective of age.
I may be misunderstanding the question, but the idea of it being powerful for a given age of child makes me slightly wonder if you might be seeing taking children seriously as something we do in order to create a particular outcome, in service of an agenda we have (that is independent of the child’s wishes). Having such an agenda for someone would not taking them seriously as a person in their own right. We do not take people seriously because taking people seriously has this or that alleged effect, we take them seriously because it is right, and because not doing so tends to impede the growth of knowledge.
It is never too late to start treating people (more) decently. We are all human, and we all lack knowledge and make mistakes. When we notice we have been making a big mistake, the thing to do is to try to correct it and move forward from here. We cannot change the past, and if we get too hung up on past mistakes, we may be inadvertently wrecking the now as well.
Sarah Fitz-Claridge, Taking Children Seriously FAQ: ‘What ages is Taking Children Seriously most powerful for, or how does its value change with age? When is it almost too late?’, https://www.takingchildrenseriously.com/what-ages-is-taking-children-seriously-most-powerful-for-or-how-does-its-value-change-with-age-when-is-it-almost-too-late￼/