If you consider a child being raised a witness, they are intellectually stimulated from a young age. From day one they are brought to meetings which speak at a level much higher than most kid's programs. Encouraged to read and write, introduced to singing and music and put in an environment with frequent social interaction with strangers and groups.
This is all very good for the growth of a child intellectually, the down point is, the closer a child approch their teens, the life experiences that were once challenging are now normal and as they grow older are actually stunting their intellectual growth.
Some stop learning and coast through the rest of their life, others continue to learn but remain indoctrinated. Many continue to learn and grow and eventually find their intellectual pursuits have outgrown the religion they were raised in.
At least that's my theory.