— Age & cognition
IQ at age 18 — what's typical?
IQ scores at age 18 are age-normed — meaning your raw test performance is compared against other 18-year-olds (or the closest reference cohort), then converted to a scaled score with population mean 100 and SD 15. The "average IQ at age 18" is therefore always 100 by construction.
By 18 most cognitive abilities have either peaked or are near peak. Processing speed continues rising slightly until early 20s, then begins gentle decline.
How IQ measurement works across ages
Modern IQ tests use age-normed scoring. A 10-year-old answering correctly on items that 12-year-olds typically get right scores higher than 100 (above their age-mates). A 30-year-old answering those same items correctly scores exactly 100 (typical for their age group). The score is always a comparison to peers, not an absolute capability.
Cognitive trajectories across the lifespan
- Fluid intelligence — peaks in early 20s, gradually declines 0.5-1 IQ point per decade after
- Crystallised intelligence — rises until 60s or 70s, then slow decline
- Processing speed — peaks early 20s, steepest age-related decline of any cognitive index
- Working memory — gradual decline from mid-30s onward
- Vocabulary, judgement, applied wisdom — typically rise through mid-life
How 18 compares to other ages
- IQ at age 12 — Early adolescence
- IQ at age 15 — Mid adolescence
- IQ at age 21 — Young adult
- IQ at age 25 — Young adult
- IQ at age 30 — Adult