I’ve always wondered what “age-adjusted” IQ scores really mean in practice. From what I understand, IQ is standardized so that the average score at any age is 100, but how exactly is that done? If a 10-year-old and a 30-year-old both answer the same number of items correctly, they don’t end up with the same IQ, right?
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You’re exactly right. Same raw score, different IQ depending on age. The Stanford-Binet, WISC, and WAIS all use age-specific norm tables built from huge stratified samples (thousands of people tested at every age band). Your raw score on a subtest gets converted to a scaled score by comparing you only to others your exact age. Then those scaled scores sum into factor indexes, and finally the full scale IQ is looked up in a deviation IQ table with mean 100 and SD 15. It’s all normative, not developmental or ratio-based like the old IQ = (mental age / chronological age) × 100 formula. That’s why a 10-year-old nailing 35 out of 50 items might hit 130, but a 30-year-old needs closer to 44 for the same score. The test expects more from adults because cognitive efficiency peaks around 25–30.
Correct - raw scores are compared to age-specific norm tables, not absolute performance. A 10-year-old answering 30/50 items correctly might score IQ 120 (comparing to other 10-year-olds), while a 30-year-old with the same 30/50 raw score might be IQ 85 (comparing to other 30-year-olds). This is why IQ reflects your percentile rank within your age group, not absolute cognitive ability. Modern tests use deviation IQ where your raw score is converted to a standard score (mean=100, SD=15) based on your age cohort’s performance distribution.
It is also worth adding that the age adjustment reveals an important shift in what’s being measured across the lifespan. For children and teens, age norms track developmental progress (how far along you are compared to peers who are still developing). For adults, it shifts to measuring relative standing within a stable population, accounting for normal aging patterns where processing speed declines but crystallized knowledge often holds steady or improves. This is why some tests like the WAIS separate cognitive domains with different aging trajectories.