mentally challenged
adj. Relating to a person with a mental disability.
The music blared loudly while a group of dancers circled around clapping and shaking their hips. Some were young adults, and others were elderly.

Those who were too frail just moved their hands in time with the music, while a few seemed oblivious to the partying around them. This festive moment took place in an unusual setting. In the assisted living unit of NorthPark, a retirement community in Hollywood, a special program takes place twice a week that unites mentally challenged students from The Quest Center with senior citizens with Alzheimer's disease.
—Lourdes Rodriguez-Florido, “No need to feel down,” Sun-Sentinel, November 17, 2002
1986 (earliest)
Ted Kennedy Jr., who lost his right leg to cancer in 1973, delivered a pep talk Sunday to a Minneapolis conference sponsored by the American Cancer Society and Mount Sinai Hospital & Auxiliary…

Kennedy last year helped found Facing the Challenge, Inc., a nonprofit group that he said tries to raise people's awareness of barriers faced by "the physically and mentally challenged." He said he rejects the terms "handicapped" and "disabled."
—Dan Wascoe Jr., “Ted Kennedy Jr. says sports helped recovery,” Minneapolis Star-Tribune, April 28, 1986