Different abilities top out decades apart
Hartshorne and Germine (2015) pooled standard test data with large online samples and found peaks spread from the late teens to the late 60s. Click a marker.
Speed on simple coding and matching tasks peaks earliest, in the late teens, and declines steadily from there.
Checked against the paper's abstract and MIT and APS summaries (Oct 2026): data from nearly 50,000 online participants plus older WAIS and WMS norm samples. Peaks are broad plateaus, not sharp points.
Comparing today's 70-year-olds with today's 30-year-olds exaggerates decline
A cross-sectional study tests different people of different ages at one moment. The older group grew up with less schooling and fewer of the conditions behind the Flynn effect, so part of the gap is generational. Following the same people over years (longitudinal design, as in K. Warner Schaie's Seattle Longitudinal Study) usually shows later and gentler decline, although repeat testing adds its own practice boost.
One answer sheet, scored at nine ages
Give the same reasoning puzzles to people of different ages and score them against their own age band. A performance that is ordinary at 25 is outstanding at 8 and well above average at 80. That's how IQ can stay flat while abilities change.
Age-band means and SDs are invented round numbers for illustration.The younger the child, the less the score predicts
Bars show the rough strength of prediction to adult scores, as broadly reported in developmental reviews; exact correlations vary by test and sample. The age-11 figure is from the Scottish Mental Survey follow-ups (Deary and colleagues).
The Wechsler family splits the lifespan in three
Axis runs from birth to 92. Age ranges as published for each current edition. The line marks the age chosen at the top. Ages 16 and 6–7 overlap between batteries; examiners choose by expected ability and purpose.
- Hartshorne, J. K., & Germine, L. T. (2015). When does cognitive functioning peak? The asynchronous rise and fall of different cognitive abilities across the life span. Psychological Science, 26(4), 433–443.
- Schaie, K. W. (2005). Developmental Influences on Adult Intelligence: The Seattle Longitudinal Study. Oxford University Press.
- Salthouse, T. A. (2009). When does age-related cognitive decline begin? Neurobiology of Aging, 30(4), 507–514.
- Deary, I. J., Whiteman, M. C., Starr, J. M., Whalley, L. J., & Fox, H. C. (2004). The impact of childhood intelligence on later life: Following up the Scottish Mental Surveys of 1932 and 1947. Journal of Personality and Social Psychology, 86(1), 130–147.
- Horn, J. L., & Cattell, R. B. (1967). Age differences in fluid and crystallized intelligence. Acta Psychologica, 26, 107–129.
- Wechsler, D. (2008, 2012, 2014). WAIS-IV, WPPSI-IV and WISC-V Technical Manuals. Pearson. Age ranges.