Brain cells grow in size and degree of myelination as a child grows from birth to adulthood. Although the number of neurons does not increase after infancy, the number of glial cells does increase. An adult brain is approximately three times as heavy as it was at birth. Between ages 20 and 60, the brain loses approximately 0.033 to 0.10 ounces (1 to 3 grams) a year as neurons die and are not replaced. After age 60 the annual rate of shrinkage increases to 0.10 to 0.143 ounces (3 to 4 grams) per year.
The brain is comprised of four main regions: the brain stem (pons and medulla oblongata), cerebellum, diencephalon (includes the thalamus and hypothalamus), and cerebrum, which includes the lobes, sulci, and gyri. (From Cohen, B.J. Medical Terminology. 4th Ed. Philadelphia: Lippincott, Williams & Wilkins, 2003.)