"What is a normal IQ for a 15-year-old?" or "What is an average IQ at age 40?" People search for these questions all the time. The answer is simpler than most expect: the average IQ for every age group is 100. It is the same for a 7-year-old child, a 15-year-old teen, and a 45-year-old adult.
This might sound strange. A 15-year-old clearly knows far more and solves much harder problems than a 7-year-old. Here is why this is not a contradiction, what an IQ score actually measures, and what really changes as you age.
Does IQ change with age?
No. IQ tests are normed by age group. That means your score is calculated by comparing you to your own age group, not to all humanity.
A 15-year-old's raw score (the number of correct answers) on a test is much higher than an 8-year-old's raw score. Yet both can have an IQ score of 100, because each person sits right in the middle of their own peers.
That is why a literal "IQ chart by age" does not really exist. The real question is: where does an IQ score place you within that specific age group?
Why average IQ is always 100 at any age
This comes down to how IQ is calculated. Historically, two different methods have been used.
Ratio IQ: an outdated method
Early IQ tests used this formula to calculate scores:
IQ = (Mental Age ÷ Chronological Age) × 100
A 10-year-old child performing at the level of a 12-year-old would score (12 ÷ 10) × 100 = 120. This method worked well for children because cognitive development progresses rapidly with age.
However, it failed for adults. What should a 40-year-old's "mental age" be? Because cognitive development plateaus in late adolescence, this formula artificially lowered everyone's IQ as they got older. To learn more about this concept, read our article on what mental age is and how it is calculated.
Deviation IQ: the modern standard
All modern tests (Wechsler, Stanford-Binet, Raven) use deviation IQ. There is no division here. Instead, test publishers follow these steps:
- The test is given to a large sample of people across each age group.
- The distribution of raw scores for that age group is plotted.
- The scores are scaled so the average is 100 and the standard deviation is 15.
A score of 100 does not measure total knowledge. It shows a position: you are right at the average for your age group. For more calculation details, check out our guide on how IQ scores are calculated.
Raw scores increase with age, IQ scores do not
This is where most of the confusion comes from. People mix up two different numbers:
- Raw score: The number of correct answers on the test. This actually rises from childhood through adolescence.
- IQ score: Where your raw score sits relative to your age group. For a person with average development, this remains around 100 throughout life.
Here is a concrete example: an 8-year-old child might answer 20 questions correctly and get an IQ of 100. At age 15, the same child answers 45 questions correctly on the test, but their IQ is still 100 because their peers progressed at the same rate.
IQ score ranges and what they mean
What each score range represents is identical for all ages:
| IQ Range | Classification | Approximate Population % |
|---|---|---|
| 130 and above | Very superior | 2% |
| 120–129 | Superior | 7% |
| 110–119 | High average | 16% |
| 90–109 | Average | 50% |
| 80–89 | Low average | 16% |
| 70–79 | Borderline | 7% |
| Below 70 | Extremely low | 2% |
The middle row is the most important: half of all people score between 90 and 109. To explore score ranges in more detail, see our guide on what a normal IQ score is and what IQ levels mean.
Why IQ test scores fluctuate more in children
Adult IQ scores stay stable over time. For children, scores can fluctuate for several clear reasons:
- Developmental speed varies. An early bloomer might be ahead at age 7, but peers may catch up by age 12. Their score seems to drop, but cognitive abilities did not regress.
- Test-day factors matter more. Fatigue, anxiety, or distractions affect children far more than adults.
- Measurement error exists. Even professional tests report scores with a confidence interval rather than a single fixed number — for example, "112 with a 95% confidence interval of 106–118".
For this reason, a single IQ score in early childhood should never be treated as a permanent label. We explain how testing works for young people in our article on intelligence testing in children.
What specific IQ scores mean
Here is how common IQ scores fit into the standard scale:
- 100: Exactly average. You are right in the middle of your age group.
- 110: High average. Higher than most people, with only about 1 in 4 scoring above this.
- 120: Start of the superior range. About 9% of the population scores above this mark.
- 128: High end of the superior range. Higher than roughly 97% of the population. In short: yes, this is a very high score.
- 130 and above: The threshold for very superior intelligence. High IQ societies like Mensa typically set their membership requirement here.
All of these scores are independent of age. A score of 130 means the same thing for a 9-year-old child and a 50-year-old adult: both sit in the top 2% of their age group.
What actually changes with age
Even if your overall IQ score stays stable, components of intelligence change over time. Research highlights two main types of intelligence:
- Fluid intelligence: Solving novel problems, recognizing patterns, and processing speed. This peaks in your 20s and gradually declines.
- Crystallized intelligence: Accumulated knowledge, vocabulary, and experience-based reasoning. This continues to grow and stays strong into old age.
A 50-year-old might perform slower on pattern recognition than a 25-year-old, but excel in verbal reasoning and knowledge tests. This is another reason standard IQ scores are normed by age. To learn more about this distinction, read our post on fluid and crystallized intelligence.
Frequently asked questions
What is a normal IQ at age 15?
100, just like every other age. Because tests are scaled so the average for 15-year-olds is 100, any score between 90 and 109 is considered average.
Is my child's IQ high for their age?
Because IQ scores already account for age, you do not need to make adjustments. A score above 110 means your child is above average compared to peers. A score over 130 puts them in the top 2%.
Does IQ decrease with age?
Standard IQ scores usually do not decrease because they are normed against your age group. Fluid intelligence slows down naturally over time, but age-based scaling accounts for this shift.
Do online IQ tests calculate scores by age?
Quality varies by test. Very few free tests use age-specific norm tables like clinical tests do. Most score results against a general population model. We discuss test accuracy in our post on how reliable online IQ tests are.
What should I do for an official result?
If you need an official evaluation for school placement or gifted identification, seek individual testing administered by a professional, such as WISC or WAIS. For a general estimate, take our free IQ test to see how your score compares to the average.