Human Brain Development

The nervous system starts to develop by the 16th day after conception, reaches 90% its adult weigh by the age of three years

After 20 weeks of gestation, the brain is about 3½ oz. At birth, the brain weighs about 14oz. At the age of 3 years, it weighs about 90% the adult weight. The average adult brain weighs about 3 pounds.

At the age of 8 months, an infant has twice as many neuronal connections as in adult. Between 10 and 18 months of age, the baby's emotions start to develop. This area is same as used for long term memory.
Between the age of 3 and 10 years, brain develops at a slower rate, still is twice as active as an adult brain. This time span is known as learning years. The brain processes the information, keeps what is useful, discards what it considers useless. This is specially true in processing sounds (music, language). The area of brain that processes the sounds, is the same area used for mathematics and spatial reasoning. If at home, a Hindi speaking person is married to an American, it will keep the information for both the languages considering them as useful including the accent. After the age of 12, child can learn another language but with difficulty in learning the accent.

After the age of 20, part of the brain that makes us feel content, starts to decline at about 15% per decade causing mid-life crisis (depression). Another part of the brain responsible for arousal also starts to decline after the age 20 at about 6% per decade.