important for what? many things occur in one's life could be very important for something!
for example, first seven years after marriage is the most important time for someone's life (is that right?)
so, this is not well defined question. First twelve year of one's life maybe important for develop one's personality, that is maybe true.
It's important in some aspects like the development of frameworks, concrete concepts about the world etc. Also important in terms of physical development.