Medieval western Europe certainly did not have the medical categories we perceive today. Kanner and Asperger's tandem research on what we now call autism, for example, was done in the 1930s and 40s. What medieval people did have was observation: an awareness that certain people acted differently. Whereas today cognitive and psychiatric conditions are diagnoses by medical professionals (or, I suppose, by Dr. Google), in the Middle Ages it would have been a process of social recognition. At what point did a person's difference render it difficult or impossible for them to fit into normal society?
For the most part, we know, people whose disabilities rendered them unable to function on their own were the responsibility of their families. We have a few snatches of stories from saints' miracle collections where the healing miracle gives us one or two sentences about the previous condition. These stories often involve siblings transporting the person to the shrine, or a mother describing how she used to help her adult child get dressed every day.
Only at the very end of the Middle Ages, the 15th century, do we see the first formal establishment of institutions dedicated to people with cognitive/mental disabilities in the West. It's possible that there would have been some similarly disabled people residing in earlier monastic hospitals along with the indigent and indigent (physically) sick, but not on a focused basis. To be sure, these 15C institutions were not hospitals or sanitoria in the modern sense, with a goal of treatment. It was just a place for them to live and maybe learn a little bit about God.
We learn a little more about the roles of the cognitively impaired in medieval society through law codes and religious sources. Already in late antiquity, Roman civil law distinguished crimes committed by someone unwell, that is, "insane"--with fuller descriptions making it clear that these laws typically applied to criminals with cognitive impairment, not the temporary insanity we all know from Law & Order.
From the high Middle Ages, it used to be standard scholarly practice to assume mental disability was interpreted as demonic possession. We now know that's far from a blanket assertion. The 13th century holy woman Christina Mirabilis (Christina the Astonishing), for example, is sometimes pointed to as a (non-canonized) saint whose hagiography pretty clearly depicts her as cognitively different than her peers, even after accounting for medieval hagiographic topoi. (The part where she climbs trees and chirps like a bird, for example.) In other cases, yes, mental illness might be perceived as a demonic intrustion, especially if it developed later in life rather than being present from birth. There are some cases of nuns described in a manner reminiscent of various mental illnesses who are denied the Eucharist because of it!
But heaven and hell certainly weren't the only options--or even, oddly enough, the primary concern. Indeed, where we see authorities intervening in cognitive health isn't to perform exorcisms and certainly not to force recantation of heresy. No, it's to regulate ownership and inheritance of property!
In late medieval into early modern England, for example, "idiots" and "lunatics" (those are the terms) were designated wards of the king--similar to minor children who were considered unfit to make legal decisions for themselves. This wasn't to ensure they were cared for; this gave the crown control over any property they owned. As a result, we have some really fascinating records of court cases where people sue to have themselves ruled "not an idiot". More frequently and sadly, though, we see family members in court on the cognitively impaired person's behalf, basically using them as a pawn in order to maintain control of property within one lineage or seize it for their spouse's extended family.
Further reading:
There is a modern strain of historiographical thought that discusses pre-modern mental and physical disability in parts of Europe through a discussion of fairy belief, popular religion, and folklore. Some scholars have drawn comparisons between common characteristics of changeling births and known disorders. From some of the primary sources I have read (and some other scholars agree) changelings appear to be more often male infants. Male infants are also statistically more likely to be born with congenital disorders (proof: http://www.ncbi.nlm.nih.gov/pubmed/11745830)
This is from "Fairies and the Folklore of Disability: Changelings, Hybrids, and the Solitary Fairy" by Susan Schoon Eberly in The Good People: New Fairylore Essays ed. Peter Narvaez (1991)-- "Specific examples of changelings, solitary fairies-- both domesticated and reclusive- and of the offspring of fairy-human matings are presented that seem to offer portraits of children who were born, or who became, different as the result of identifiable congenital disorders." "Of course, not all descriptions of fairy races reflect folk remembrances of children born with congenital disorders, but there are number of fairy characters who seem to so clearly represent certain congenital disorders that they are difficult to interpret as purely the products of imagination."
This author's evidence (and my own experience with sources) seems to come mostly from the British Isles. I would also recommend "The Invisible Made Visible: The Fairy Changeling as a Folk Articulation of Failure to Thrive in Infants and Children" by Joyce Underwood Munro, in the same volume.
If I can jump on the back of your question, I'm curious what the perception of monarchs and aristocrats with these conditions was and the effect on their control over land and titles. Would reliance on regencies be the norm? Was their condition irrelevant when it came to their right to inheritance or would others contrive to pass the titles to a relative?
Would not Down Syndrome infants have a very low chance of surviving into latter childhood due to various physical issues?
The answers presented here underscore a problem with the isolated bibliographies of the various fields of the humanities/social sciences. /u/sunagainstgold calls to question the value of information gathered by folklorists from pre-modern/pre-industrial European peasant societies – a position presented by /u/GrumpyChupacabra.
For two centuries folklorists have gathered information about the oral traditions and cultural practices of these societies, much of the time working with an assumption that the folklore they were considering was largely static. It was a point of view reinforced by the discovery of folktales and legends and of evidence of cultural practices in primary sources from ancient sources clear though to the pre-modern period. The assumption of the early folklorist was not without value even though it was flawed: we now understand that oral tradition – like the larger realm of culture – is in constant change.
Pre-modern European societies seemed to be preserving ancient forms because change in those societies was glacial and appeared to be largely non-existent. This lead Jacob Grimm (1785-1863) to discuss the bits of a shattered gem scattered amongst the grass, waiting to be collected and reassembled to reveal some ancient, Germanic body of story and belief. Similarly, (but in a different century) it inspired Celtic specialist Kenneth Jackson (1909-1991) to suggest in a lecture in 1964 that it was possible to use Irish oral tradition to “peer through a window into the Iron Age.” What this line of thought concluded was problematic but not without merit. Because folklore, like all culture, changes over time, one must use caution when applying observations about the nineteenth-century folk to project backwards into a remote past. At the same time because change in folk culture appears to have been – indeed – glacial there is some merit in looking at documented folk culture to understand what might have been going on previously.
Historians by their nature consider the documentation of change, and this can set their methodology at odds with that of the folklorist. As for me, I embrace Christopher Lloyd’s call for a unified approach to the social sciences/humanities, as expressed in his 1986 volume “Explanation in Social History.” Ideally, we would use the bibliographies and methodologies of each of the disciplines in order to tease out insights about the past. Unfortunately, the bibliographies are so enormous and the methodologies have become so specialized that the ideal is largely unattainable.
At the core is the problem that it is easier to document and study change than it is cultural continuity. People discuss – and write about – the things that change more than the things that stay the same. For this reason, the methodology of the folklorist can be of value when studying cultural elements that tend to change very little over time. /u/GrumpyChupacabra was correct to cite the example of Bridget Cleary as a well-documented example of the nineteenth-century folk applying folk practice to the problem of a sick, delirious woman who seemed to me a “stock” – a magically imbued replacement for the actual woman who had been abducted. The abuse of the woman is an example of how the folk used the belief that supernatural beings abducted people and applied that belief to an immediate situation. Folk wisdom called for the abuse of the thing the fairies had left behind in order to inspire the fairies to return with the actual human.
Examples of this sort of thing appears in numerous nineteenth-century sources, often dealing with infants and young children who did not seem to thrive and were likely exhibiting one of the conditions OP questioned. Can these sources and the documentation of folk belief by folklorists in a pre-modern world be applied to a medieval period? Not without a great deal of caution and source criticism. But there is little question that these pre-modern sources can shed insight onto the problem.
Would there be a connection between the village idiots and people with intellectual disabilities?