The first five years of a child's life are far and away the most important in terms of long-term development prospects. Ninety percent of brain growth occurs during those first years. The brains of small children are also much more flexible and malleable, prone to being shaped by things like care and nurturing, but also by violence, stress and abuse. What happens in those first five years, it turns out, can have dramatic consequences in a variety of ways -- socially, cognitively and even financially.
Scientists call this state of heightened adaptability in the developing brain "plasticity" and it is what it sounds like -- a time when the brain, and thus the person to whom it is attached, can be molded and shaped to reflect the environment in which that person lives. 
Researchers say the financial benefits in offering quality preventive care at an early age can translate into huge savings down the line. It is much harder to stage effective interventions with teens, and much more expensive. For one thing, their brains are less "plastic" which means interventions take longer, have lower success rates and can be harder to keep consistent. 
http://www.insidebayarea.com/breaking-news/ci_22833772/oakland-effect-home-visitation-programs-can-increase-childs