My IQ TestAverage IQ by Age: How Thinking Abilities Change Over Time
The average IQ is 100 at every age because tests compare you only with people your own age. Underneath, though, raw abilities change: reasoning speed tends to peak in the twenties, while vocabulary and knowledge often keep growing into the sixties.
Why age-normed scores stay at 100
Modern tests convert raw answers into a score by comparing you with a reference group of the same age. A typical ten-year-old and a typical seventy-year-old both score 100, even though they answer very different numbers of questions correctly. That design is what makes it possible to compare people across the lifespan fairly.
How raw abilities change across the lifespan
Research from the Seattle Longitudinal Study and large cross-sectional studies shows two broad patterns.
| Age range | Fluid reasoning & speed | Crystallized knowledge |
|---|---|---|
| 6–12 | Rising rapidly | Rising rapidly |
| 13–19 | Still rising | Rising |
| 20–30 | Near peak | Rising |
| 30–45 | Slow, gentle decline begins | Rising or stable |
| 45–60 | Gradual decline | Near peak |
| 60–75 | Noticeable slowing | Mostly stable |
| 75+ | Larger individual differences | Gentle decline |
Childhood scores are less stable
IQ measured before about age six predicts adult scores only moderately. By adolescence, scores become much more stable, and adult scores correlate strongly over decades. The Lothian Birth Cohort studies in Scotland famously found a correlation of around 0.7 between scores at age 11 and age 77.
That still leaves room for change. Education raises scores — each additional year of schooling has been estimated to add one to five points. Our article on improving cognitive skills looks at what helps at any age.
Protecting thinking skills as you age
Physical activity, good sleep, managing blood pressure, staying socially engaged and continuing to learn are all linked to healthier cognitive aging. Hearing loss, if untreated, is one of the largest modifiable risk factors for later decline, according to the Lancet Commission on dementia.
Curious how your current reasoning compares with others? Try the free IQ test — it is scored against an adult reference group.
Common questions
What is the average IQ for a 15-year-old?
100, as for every age group, because scores are normed against peers of the same age.
Does IQ decline with age?
Age-normed IQ stays fairly stable, but raw processing speed and fluid reasoning gradually decline from middle age, while knowledge often holds up well.
Can you increase your IQ as an adult?
Modestly. Education and practice on reasoning tasks can raise scores a few points, though large permanent gains from brain-training apps are not well supported.
About the author
Dr. Elena Marsh — Elena has spent over a decade studying intelligence measurement and test construction, with a focus on fluid reasoning and fair item design.
Reviewed by Prof. Daniel Okafor — Daniel reviews item statistics and norming assumptions for our test and guides.
Sources and further reading
- Schaie, K. W. (2013). Developmental Influences on Adult Intelligence: The Seattle Longitudinal Study (2nd ed.). Oxford University Press.
- Deary, I. J., et al. (2004). The impact of childhood intelligence on later life. Journal of Personality and Social Psychology, 86(1), 130–147.
- Ritchie, S. J., & Tucker-Drob, E. M. (2018). How much does education improve intelligence? Psychological Science, 29(8), 1358–1369.
- Livingston, G., et al. (2020). Dementia prevention, intervention, and care. The Lancet, 396, 413–446.