Mental age is a concept related to intelligence. It looks at how a specific child, at a specific age—usually today, now—performs intellectually, compared to average intellectual performance for that physical age, measured in years. The physical age of the child is compared to the intellectual performance of the child, based on performance in tests and live assessments by a psychologist. Scores achieved by the child in question are compared to scores in the middle of a bell curve for children of the same age 
However, mental age varies according to what kind of intelligence is measured. A child's intellectual age can be average for his physical age but the same child's emotional intelligence can be immature for his physical age. In this psychologists often remark girls are more emotionally mature than boys in the tween years. Also, a six-year-old child intellectually gifted in Piaget terms can remain a three-year-old child in terms of emotional maturity. Mental age was once considered a controversial concept.
During much of the nineteenth century, theories of intelligence focused on measuring the size of human skulls. Anthropologist well known for their attempts in correlating cranial size and capacity with intellectual potential are Samuel Morton and Paul Broca.
The modern theories of intelligence began to emerge along with experimental psychology. This is when much of psychology was moving from philosophical to more biology and medical science basis. In 1890, James Cattell published what some consider the first "mental test". Cattell was more focused on heredity rather than environment. This spurs much of the debate about the nature of intelligence.
Mental age was first defined by the French psychologist Alfred Binet, who introduced the intelligence test in 1905, with the assistance of Theodore Simon. Binet's experiments on French schoolchildren laid the framework for future experiments into the mind throughout the Twentieth Century. He created an experiment that was designed as a test to be completed quickly and was taken by various ages of children. As was expected, the older children performed better on these tests. However, the younger children who had exceeded the average of their peers were said to have a higher "mental age" and those who performed below average were deemed to have a lower mental age. Binet's theories suggested that while mental age was a useful indicator, it was by no means permanently affixed and individual growth or decline could be attributed to changes in teaching methods and experiences.
Henry Herbert Goddard was the first psychologist to bring Binet's test to the United States. Goddard was amongst one of the many psychologists in the 1910s that believed intelligence was a fixed quantity. While Binet believed this wasn't the case, the majority of those in the U.S believed it was hereditary.
The limitations of the Stanford-Binet caused David Wechsler to publish the Wechsler Adult Intelligence Scale (WAIS) in 1955. These two tests were split into two different ones for children. The WAIS-IV is the known current publication of the test for adults. The reason for this test was to score the individual and compare it to others of the same age group rather than to score by chronological age and mental age. The fixed average is 100 and the normal range is between 85 and 115. This is a standard currently used and is used in the Stanford-Binet test as well.
No matter what the child's chronological age, if the mental age is the same as the chronological age, then the IQ will equal 100.
Modern intelligence tests, including the current Stanford-Binet test, no longer compute scores using the IQ formula. Instead, intelligence tests give a score that reflects how far the person's performance deviates from the average performance of others who are the same age, arbitrarily defined as an average score of 100.
Regarding the theory of mental age divided by physical age x 100. This is incorrect. Otherwise regular people would have IQ's of 250 normally, and Einsteins IQ is 160.
An IQ or intelligence quotient, is a score derived from one of several different standardized tests designed to assess intelligence. The mental age is the age group which scored such a result on average.
Mental age, as well as IQ have limitations. Binet did not believe these measures should be used for a single, permanent and inborn level of intelligence. He stressed the limitation of the test because intelligence overall is too broad to be represented by a single number. It is influenced by many factors such as the individual's background and changes over time.
Throughout much of the 20th century, many psychologists believed intelligence was fixed and hereditary while others believed other factors would affect intelligence.
After World War I, the concept of intelligence as fixed, hereditary, and unchangeable became the dominant theory within the experimental psychological community. By the mid-1930s, there was no longer agreement among researchers on whether or not intelligence was hereditary. There are still recurring debates about the influence of environment and heredity upon an individual's intelligence and the intelligence intentional.