In 1981, Rav Moshe Feinstein zt”l extensively outlined the extent of the obligation to educate a child with special needs. Rav Moshe explained that those with diminished mental capacities, who may not fully comprehend all things but nonetheless have some intelligence, are required to observe at least certain mitzvot as adults. Accordingly, Rav Moshe held that we are obligated in the mitzvah of Chinuch, educating them when they are children.