I believe that two people of a similar age, who both don't posses any learning difficulties, can only be distinguished in intelligence from one another through detecting psychological qualities.
Example; Let's have two 20 year old male people; Person A and Person B
Person A is said to be smarter than person B, scores higher academically, solves puzzles faster et cetera.
Person B how ever has an unknown fear of failure and constantly sets him self up to fail.
https://en.wikipedia.org/wiki/Setting_up_to_fail
It is easy for me to see how people would so quickly think that person B is not in fact as intellectual as person A
Edit: This example does not have to be referred to throughout the discussion, the question is more important, and more examples may be used.
Example; Let's have two 20 year old male people; Person A and Person B
Person A is said to be smarter than person B, scores higher academically, solves puzzles faster et cetera.
Person B how ever has an unknown fear of failure and constantly sets him self up to fail.
https://en.wikipedia.org/wiki/Setting_up_to_fail
It is easy for me to see how people would so quickly think that person B is not in fact as intellectual as person A
Edit: This example does not have to be referred to throughout the discussion, the question is more important, and more examples may be used.