Charlie Gordon's main problem is mental deceleration. In the book, which was published in 1966, the terminology that would have applied would have been "mental retardation". We no longer use that term, and instead, we use cognitive limitation, or cognitive deceleration, because it entails that the individual has not been able to develop in tandem with the cognitive expectations of the chronological age.
Charlie tells us about this from the very start. We also find out that he is part of an experimental program that aims to use him as a research subject to perform a brain operation that would make him accelerate (the opposite of decelerate) cognitively. The outcome would be that Charlie would "become smart".
I rite compushishens in Miss Kinnians class at the beekmin collidge center for retarted adults where I go to lern 3 times a week on my time off. Dr Strauss says to rite a lot evrything I think and evrything that happins to me but I cant think anymor because I have nothing to rite so I will close for today...
We also learn that Algernon, the rat first used to perform the same operation experimentally, will increase its intelligence but will eventually lose all of its skills and die. Charlie knows that he will only get a brief taste of life at the other side of his spectrum; that he will not be intelligent forever. Sadly, he will get to know the world from both sides and somehow he will have to fit in the middle. It is hard to do so, especially when life as a mentally challenged individual is right around the corner again. He will be the human Algernon. That is like living with a death sentence in life.