At what age does cognitive decline start?
Some aspects of cognitive performance start declining as early as age 25 — far earlier than most people expect. But "cognitive decline" is not one uniform process: different abilities peak and decline at very different ages.
| Cognitive ability | Peaks at | Decline begins |
|---|---|---|
| Processing speed | ~20 years | ~25 years — fastest decline |
| Working memory span | ~25 years | ~25–30 years |
| Episodic memory | ~25 years | ~30 years |
| Fluid intelligence (reasoning) | ~25 years | ~30 years |
| Crystallised intelligence (vocabulary) | ~50s | ~60–65 years |
| Emotional regulation | ~50–60 years | Very slow decline |
The nuance: Gradual, measurable decline in processing speed from age 25 is very different from the functional impairment of dementia. Most people don't notice any real-world effect until their 60s–70s, when multiple domains decline simultaneously. The brain's compensatory capacity is enormous.
The Reaction Time test is one of the most sensitive measures of processing speed decline — taking it annually gives you an objective trend line. The Number Memory test tracks working memory span changes over time.
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Quick Answer
Processing speed and working memory begin declining around age 25. More complex abilities like vocabulary and crystallised knowledge peak in the 50s. Most people don't notice decline until their 60s-70s, when multiple domains decline together.
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