In my view, I think child's most significant education impact is right from learning from their parents and family members.

The family is more significant. The mother, father and siblings are the first teachers before the child reach the school going age. They teach the child what is good and what is bad. What the child should do and what he/she should not do. The family plays a very important role

I would say it is as they turn into year three. That is as they shift from "learning to read" into "reading to learn". You can test my opinion on the basis that, reading is pivotal to knowledge building. Yet, research suggest that people who develop a reading habit early are likely to keep it. And it is my perspective which is based on 15 years of teaching.

The formative years. Between 4-11 years old