Wiktionary
a. (context politically correct English) having a low intelligence; having some mental illness
Usage examples of "mentally challenged".
Children from Larak's age to sixteen, who would be physically and mentally challenged by the opportunity of 'unstructured' classes were asked to enrol.
The lead character, Edward, is considered mentally challenged—.
Even a mentally challenged shark would figure out that sea turtles did not wear boxer shorts printed in flying piggies, and no turtle would be yattering streams of obscenities between chain-smoker gasps of breath.
CNN had a story, but like a lot of CNN stuff, most of it seemed to have been garbled by a mentally challenged paranoiac.
The Burg has had its share of exhibitionists, some mentally challenged, some drunk beyond reason, some just out for a good time.
The indulgent father who can't deny a favorite, if mentally challenged, child.
Thus it was when I purchased a package of Junior Mints prior to exposing myself to the cinematic potentials of Lost Souls, I did so with the idea of lowering my analytic capacity to that of a mentally challenged person for whom English was a second language.