The determinations of which mental handicaps translate into which halachic standings are too complicated and individualized for this forum. Rather, we will use the term “mentally challenged” for whoever it is that meets your halachic assumptions. We will touch on the laws regarding a cheresh and a shoteh (loosely translated as deaf-mute and lunatic, respectively) and a child, who are often bunched together as being exempt from mitzvot.[1] Although we will not deal with your questions in an exhaustive manner, we hope to add perspective and provide some guidelines.
It is inaccurate to say that the Torah’s laws do not apply to those who are exempt from mitzvot. For example, it is forbidden to feed non-kosher food to a child,[2] a cheresh or a shoteh.[3] Rather, they are personally exempt from responsibility for mitzvot because they lack the level of understanding that such commitment requires.[4] Since people who are exempt from a mitzva receive reward for fulfilling it,[5] one might posit that there is intrinsic value in the mentally challenged performing mitzvot. However, a person who is mentally challenged is less connected to the mitzvot than someone with an incidental exemption because his actions lack the level of cognitive intent that others have.[6]
On the other hand, although a minor also lacks full intent,[7] he is nonetheless instructed to perform those mitzvot he is capable of.[8] Furthermore, he does the mitzvot with berachot, to which we answer amen.[9] (It is forbidden to answer amen to an unauthorized beracha.[10]) A possible explanation is that since there is a mandate to train him to perform mitzvot (chinuch), his mitzvot and berachot have halachic standing. In contrast, the mitzvot and berachot of one whose prognosis is that he will never be obligated in mitzvot (and apparently free of the chinuch mandate)[11] would lack such standing. However, a different possibility is that the mitzvot and berachot of anyone who is capable of carrying them out with a basic understanding of He Who commands and what He commands are significant.
A test case is a child under the age of chinuch[12] who is able to make a beracha with reasonable understanding. The Mishna Berura[13] and the Yabia Omer[14] say that one should not answer amen. However, Rav Bakshi-Doron[15] notes that most people answer amen to preschoolers’ berachot and substantiates the practice by noting that the child’s basic understanding of his words’ purpose makes this appropriate. He states that the same is likely true for a cheresh and a shoteh. Reportedly, Rav S.Z. Auerbach would inconspicuously answer “ame_”, without the “n,” to small children’s berachot.[16]
Rav Auerbach has two important ideas that enable one to teach and respond to berachot of the mentally challenged: First, he says that many if not most of them should not be considered like a shoteh, whose problems are psychological, or a cheresh, who has a specific condition the Torah addresses. Rather, a mentally challenged person is analogous to a child. A person of bar mitzva age who is on the level of a pa’ot (an average six-year-old or so) is obligated in mitzvot, although he is not culpable like others.[17] Thus, one who can be expected to reach that level should receive training, even when he is a child.[18] Rav Auerbach is also quoted as saying that one can use HaShem’s Name in teaching those mentally challenged who are on an even lower cognitive level (but not to answer amen) because their ability to make berachot has value in that it enables them to fit into their surroundings better.[19]