Can one with Mild MR fail to be diagnosed before the age of 18?

First off we all say "of course". Yet this leads us to the second question, if one is not diagnosed until after 18 how can we consider them MR if there age of onset is undetermined??? But what does this mean then....
It is called the "Forest Gump Question".Click here to view a presentation on this Gump question. Can one succeed to the best of their ability if accommodations, modifications and an IEP are not in place? One tends to say "no", but why is this happening? One reason could be the student was able to coast by with low average grades, the other would be that in the late 70's to 80's many school psychs did not want to diagnose to avoid parental pain, also what about the question of over-diagnosing a specific population?

It is well-established that schools were and are reluctant to classify children as mentally retarded, particularly African-American students since the 1970s (MacMillan & Siperstein, 2002). Schools frequently assign a more ‘‘palatable’’ label to students who would otherwise be classified as mentally retarded, using labels such as ‘‘specific learning disability’’ or ‘‘speech and language impairment.’’ In Atkins cases, this frequently works against the defense’s efforts because there is no developmental history of an individual ever being diagnosed as mentally retarded, thereby making it difficult to prove the developmental criterion of mental retardation.(p.9).

Gresham, F. M. (2009). Interpretation of Intelligence Test Scores in Atkins Cases: Conceptual and Psychometric Issues. Applied Neuropsychology, 16, 91-97

I am asking these questions because they are going to play into a post that will be up soon discussing the misdiagnosis and the issue of criminal justice and the death penalty. Please stay tuned and give me your thoughts.