The brain takes the longest of all human organs to reach maturity. Human brains do most of their growing in the first five years of life, quadrupling from birth size to 95 percent of adult size. Many neural connections are made during these years, which continue to affect our brain functions for the rest of our lives. Most of the neurons that make the connections in our brains to help us move, sense and think already are formed before we're born, but millions more continue forming after we're born [source: American Museum of Natural History].
Studies have found recently that it's the speed at which the brain grows -- rather than how large it becomes -- that predicts how smart an infant will become as an adult [source: Rettner]. Researchers using magnetic resonance imaging (MRI) scans looked inside premature babies' brains right after they were born and until they reached what would have been full term, and then gave the children intelligence tests when they were 2 and 6 years old. Children whose cerebral cortices had grown about 5 or 10 percent less than those of other infants in the study scored lower on intelligence tests when they turned age 6.
Comparisons of MRI findings in baby chimpanzees with those of baby humans also have helped understand how human brains grow in volume much more rapidly and sooner than those of chimpanzees, even though humans and chimps start life with undeveloped forebrains. The study has shown that white matter in the brain's prefrontal cortex grew much more rapidly in infant children than in infant chimps. This might explain why humans have far better communication and social interaction shills than chimps [source: Bhando].
Some parts of brain development continue through the teen years, and ongoing learning in adult life can also help parts of your brain get larger, as well as smarter. One study even found that the brains of some London cab drivers grew as they learned more complicated routes.
In addition to mental activities stimulating brain growth, studies also have confirmed that exercise can promote new stem cells in the brain to grow and develop. The researchers used mouse models to test theories about brain chemicals and stem cells and found that new nerve cells survived and grew by levels of nearly 190 percent in middle-aged mice who exercised. Physical activity also helped younger mice [source: Science Daily].
The key then, is that our brains might not continue growing in terms of mass or size after we reach adulthood, but they continue reorganizing and developing throughout most of our lives.