Your brain began to develop in a matter of days to weeks after conception. The main sections of brain tissue were visible by the seventh week of gestation. And look what you have today!
No surprise, in a sense the brain matures more slowly than the physical body. The corpus callosum, one of the three main bridges containing 80-250 million fibers that allows the right and left hemispheres to talk with each other, isn’t myelinated (paved with brain asphalt, to use a highway metaphor) until age 20 or 21.
Development of the frontal lobes is complete only in adulthood, perhaps at age 20 or 21. While development of the pre-frontal lobes may be complete around the mid twenties, by age 25-27.
In addition, it may require an additional 1.6 years for the male brain to mature.
Hopefully, the information in these Brain References will help you understand more about brain development before and after birth.