coming to rajkumar character he is mentally challenged person i.e brain 6yrs kid in 25-30 yrs old man body i.e brain disability.actually there is no difference between words mentally challenged n retarted.just twist of words.mentally challenged is polite way of referring to someone who has brain disability as retarded is considered offensive.
for eg there r some politically correct terms used these days
visually challenged
physically challenged
petroleum transfer engineer (d man at petrol station who fills petrol in d cars) etc..
nowadays d words mentally challenged is used for person who has brain disability n don't know wat is right n wrong n no fault of theirs.
whereas retarded is used for person who know d difference btwn right n wrong n still does d wrong.