Boy, all this fuss
But, seriously, people are intelligent because they usually want to be. Age typically has nothing to do with level of intelligence; its usually used for referencing how a human's brain goes through a logical thought process and the age when the brain usually starts to use logic is about 13 but that can differ from person to person.
It is a sad thing, but even today, many adults still fail to use proper logic. Many people use subjective logic where they justify something in their mind to be either right or wrong without basing it on any real ethical values.
I would also like to state that I am proud of the knowledge I have even though not all of it is necessarilly useful.