When Do Juveniles' Brains Stop Growing?

Brain Structure

• The brain is composed of structures such as nerve cells, or neurons, the connections between them -- called synapses -- and blood vessels. At birth, a baby has billions of neurons. A newborn̵7;s brain is about one-quarter as large as that of an adult̵7;s. By the time a child reaches the age of three, her brain is about 80 percent of adult size; by age 5, it has reached 90 percent of adult size. The growth results from an increase in the size but not the number of neurons, which grow in long structures called dendrites. However, the synapses increase in number as the child grows.

Learning, Processing and Pruning

• Brain development depends on activities and experiences. The neural circuits use electricity to process information. Each time the child engages in an activity such as talking, walking, reading a book or riding a bicycle, the brain activates neural circuits. Repetition of these activities strengthens the circuits -- called neural pathways ̵1; and unused circuits gradually wither. This pruning process makes the brain operate more quickly and efficiently. When a child receives little stimulation, less brain development occurs. As a child grows older, critical developmental windows close, and the brain cannot make up for development that did not occur at the earlier stage. The critical window for learning a second language, for example, is roughly between 5 years of age and puberty.

Brain Growth

• Nutrition is particularly important for brain growth and affects the ultimate size of a child̵7;s brain. Babies not well nourished in the womb and children who are poorly nourished for the first two years after birth may have smaller brains because the structures in the brain don̵7;t have the right material to develop properly. Fat, for example, is vitally important to the myelin, or coating, of the nerve sheath. Toxic substances such as alcohol and infections can also affect brain growth.

The Frontal Lobe

• During adolescence, the brain, like other parts of a teenager̵7;s body, goes through a growth spurt. The area called the frontal lobe -- because it is the area in the front half of the brain -- governs impulse control, planning and reasoning. The teen brain develops more myelin as it ages, and the last area of increased myelin is the frontal lobes. Physically, the teen brain may be the size of an adult̵7;s, but until development in the frontal lobe is complete -- which does not happen until most people reach the early 20s -- the brain is not yet fully grown.