I apologise if the phrase "mentally challenged" was offensive or inappropriate (the English language never fails to trip me up). As far as I can recall, the movie does not identify the specific developmental disability, but the main character engages in obsessive and ritualistic behaviours and has a number of challenges with fully integrating in society. His friend has different issues. The movie has a very positive message, as had the OP. I encourage you to judge by yourself rather than read the IMDB description. If you feel I have misrepresented it, I am interested in your views, my contact details are in my profile.
The real issue, I thought, is what is the appropriate level of state intervention / social support in circumstances where the parents-caregivers pass on.