I keep seeing people ask what the average IQ is for a 12 or 13 or 14 year old as if there is a different number for each age. But I thought IQ was always normed to 100 regardless of age. So what is actually going on when people talk about average IQ by age and does cognitive ability actually develop differently at different ages during childhood and adolescence?
The question makes total sense intuitively but the norming system is specifically designed to make it unanswerable 
Asking what is the average IQ for a 13 year old is like asking what is the average percentile for a 13 year old. It is always 50th by construction. The more interesting question is how different cognitive abilities develop at different rates across childhood and adolescence.
This is a really common misconception and it is worth clearing up 
Modern IQ scores are always age normed by design, meaning your score of 100 at age 12 and your score of 100 at age 40 both mean the same thing: you performed at the average level for your specific age group. So there is no meaningful answer to what is the average IQ for a 12 year old because the average is always 100 by definition regardless of age. What does change with age is raw cognitive performance. A 14 year old will outperform a 10 year old on almost every cognitive task simply because the brain is still developing. But IQ scores adjust for this by comparing you only to others your own age, which is why a 12 year old and a 35 year old can both score 105 even though their absolute cognitive performance looks completely different.
What does genuinely vary by age is the trajectory of different ability types. Fluid reasoning peaks in the mid to late 20s and then gradually declines. Crystallized intelligence keeps growing well into middle age. Processing speed starts declining relatively early. A well designed assessment that separates these domains like RIOT IQ gives you a much cleaner picture of where you actually stand across different ability types 
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