@ASC
Mental impairments boil down to atoms and energy: they are a condition that can be described, diagnosed, and theoretically treated through a materialist/scientific approach. The soul is supernatural and - by definition - can’t be described or understood through materialistic methods since a soul is immaterial.
So yes, a person with down syndrome - which is simply a physical condition that affects the body/brain - can go to hell. If a person is capable of going to heaven - which necessities the consent of the creature - then they are always capable of going to hell. There is no such thing as being capable of going to heaven but not being capable of going to hell. If somebody claims that a person with down syndrome can’t go to hell , then they are claiming that they aren’t made in the image of God and therefore dehumanizing them and putting them on level with an animal.
Peace.