It just so happens that I have been listening to this lecture about Waldorf education, and this guy talks about how they see the evolution of children.
They say, the physical body is born at birth, this seems obvious. But the vital body, etheral body as they call it, (pranamaya kosha) is not totally born, meaning separate from the mother's until 7 years of age. This second birth is said to be manifested physically by the change of teeth. The "astral" body what is closest of what usually called soul, the emotional "desire" body is also attached to that of the parents', until the age of 14, and this third "birth" manifests as puberty. The fourth birth would be when the Ego is free of any parental influence, the man or woman is a grown up, an individual with the sense of being someone. This is set to the age of 21.
This theory (as I am uncapable to verify it by spiritual sight as I lack that) seems credible to my intuition ... when I was in 4th year of college, and thought back to my life, anything before 21-22 was like a dream, like I was sleeping and somehow, non existent.
Now, as antroposophy teaches, this is not where evolution stops, altough modern science thinks it does. Modern science is too focused on cantitative and perceptual knowledge to acknowledge individual growth through intuition, emotion or morality.
But antroposophy says our goal is to further evolve our physical, vital and astral bodies to the level where all these are spiritualized.
So I think, because I don't know teachings regarding this, that the evolution of a human continues, with a qualitative change, or jump every seven years, even if these are not as obvious or important that the first three ones. These are slower and not always outwardly apparent changes. The reason would be that all living humans did succed throughout history and several incarnations to develop the physical, vital, astral bodies and the ego, and the evoution of a child just repeats this in a much shorter timeframe. What is yet to be done, this is what we work on between 21 and the end of our lives, and we build our future "selves" brick by brick, day by day, experience by experience.
So the answer to your question is function of what do we mean by soul, and what is that soul's journey, because there are several "births" even if one lifetime.
For example, I feel that during 2007 (7x 5 = 35 years old), I passed through a crysis and obstacles what changed me in a way that it can be called a new birth itself.
PS. These births are not to be seen as if before them the ego, astral or vital bodies are non-existent, but rather than unseparated of the coresponding worlds (lokas). Children seem to maintain a spiritual sight until the age of three (ocassionally to five), and if we watch carefully, we realize that often they know things they should not know by ordinary perception or senses. For example they seem to have a keen emotional intuition, immediatly knowing how their parents or even strangers feel.