An individual’s reading habits and ability says a lot about their intelligence. High IQ’s tend to have a thirst for knowledge. Compared to video and audio, reading is the most efficient method to acquire said knowledge.
"At the highest end of this scale of intelligence, a child may learn to read in just a few months, less than a year. Reading above your age group typically denotes a higher verbal IQ. This is partly why vocabulary is so highly correlated with g, because those who are more verbally precocious read earlier and read more books and so learn more words.
By ‘learning to read quickly’, you mean the speed at which one reads words at rather than the age of which one learnt to read then that is also an indicator; it’s an indicator of processing speed — which is how fast you can process and then respond to information.
The average reading speed of a person is 200 words per minute. The top readers, who’ve had a lot of practice and presumably are uneven in their cognitive profile allowing for incredibly good processing speed, read about 1000 words per minute. Since processing speed has a high enough correlation with g that it’s an index on IQ tests, it’s a semi-reliable measure. I’ve read an account of someone who’s profoundly gifted who reads at 800 words per minute. In general, most gifted people would read faster than 200wpm but not that much faster, say 300wpm or 400wpm.
By reading above your age, this is highly indicative when talking about non-fiction. A young teenager, say around 12 years old, may have an interest in physics and so start reading up on it. They may also gain an interest on psychology and so browse around the internet, reading studies. They may gain an interest in economics and learn the basics then get more specific and then teach themselves by observing others on how trading works.
Reading above one’s age is indicative of verbal IQ, which has a 1.0 correlation with g — the highest out of any other intelligences like spatial because they have noise which we don’t quite yet understand. They have the ability to logically understand the more abstract concepts, above a certain point may think in anologies emphasising their abilities. The larger vocabulary allows them to understand the words. “