The age range at which the most intensive myelination of nerve cells occur is Zero to two years. Myelin is a lipid-rich substance that surrounds the axon of some nerve cells, forming an electrically insulating layer. Myelination is part of the development process that greatly contributes to brain growth and maturation. Myelination begins early in the 3rd trimester with the most rapid period of myelination occurring in the first two years of life.