There are different types of learning disorders recognized by the DSM-IV, based on the area that is affected: Mathematics Disorder, Reading Disorder, and Disorder of Written Expression.
Each one has their own diagnostic criteria, based on the fact that a person can’t perform as well as he or she would be expected to based on chronological age. For example, the diagnostic criteria for Disorder of Written Expression is as follows.
Writing skills, as measured by individually administered standardized tests (or functional assessments of writing skills), are substantially below those expected given the person's chronological age, measured intelligence, and age-appropriate education. (behavenet.com)
You will notice that the criteria include standardized testing and mention age, intelligence, and education. For all learning disorders, the level of education a person has had access to is crucial. A person with no learning disorder may seem to perform below expectations if there is a lack of education. Also, lower intelligence is not considered a learning disorder. A person only has a learning disorder if he or she is not achieving up to his or her intelligence.
As with every entry in the DSM-IV, these are just guidelines. Individual situations and circumstances may differ, and each evaluator must carefully consider the big picture and all elements of the case.