“retarded” itself is merely a polite way replacing more insulting terms used previously
The Polite terms in contemporary English involving combing the words “Mental” or “Intellectual” with “Handicapped” or increasingly “Disabled.”
“Developmental.. ” refers to when the problem started, meaning it started from birth (as opposed to having resulted later in life (such as from disease or injury)
Bnei Torah prefer polite speech, but there is nothing wrong with an Am ha-Aretz using the word “retarded” since everyone knows such persons are intellectually challenged which due to poor training results in vulgarity, and will forgive them.