I have gone off on a tangent however, so I will return to the concept of Yetzer Hora - Yetzer Tov. This whole reasoning of how we acquired the Yetzer Tov only later in life, and why, seems very logical. Although there is an essential question which will be asked by any parent or other objective observer of children. It is here that another concept must be brought to light. A G-dlyu soul, and an Animal soul.
Animals do nice and good things.
It seems thus degrading to the animals to give our evil inclination such a status. To proceed, we must examine the difference between humans and animals. This is the key concept. It is instinct that leads a mother dog to feed her young. When man was created, he was given all of these things, existence, life and an Animal Soul.