For some of us it may be hard to know the exact method for interacting with people who are mentally handicapped. It is never too late to learn about the different methods that you need to know when you want to be respectful and also attentive to a person who will not be on the same brain wavelength as you. This is common among people who have not previously interacted with, or been in the presence of (long term) with a person who had a mental disorder or other. So here are some of the basic things that you should keep in mind.
Intelligence levels vary depending on the disorder
Generalization is one of the worst ways that you can think about a person who has a mental disorder. Just like any other disease, there are levels of mental retardation that a person who is affected by a mental disorder will have. A person born with something like autism might be harder to deal with than someone who has ADHD (attention deficit/ hyperactivity disorder). So before you make any comments or start talking very slowly or in baby-talk, let the other person talk first and gauge your own reaction. In most cases (especially adults), the person would have adapted to living life normally and will be able to communicate with you as any other person with a normally functionally brain.
Learn your facts
Before you go around believing rumors or thing off the internet, go for a place like a reputed child development center, or volunteer there, to understand about different brain and mental disorders that regularly wrack children. The psychiatrists and the psychologists along with other professionals who work at these locations would know their information much better than any article online.
Mentally handicapped people have their own amazing skill sets
A common known understanding among people is that mentally handicapped persons are extremely skilled at different things. Most children who are handicapped at early ages take art class for kids Hong Kong and other exercises that lets them develop a certain set of skills to extreme levels. In a general way you can view normal functional people as all-rounders while persons with mental handicaps would be geniuses in a certain field. This is because mental handicapped children (when identified early on) are diverted to learn only a limited number of things to help their brains function easily and would hone their known skills to outrageous levels as they grow.
The best thing to do when you meet anyone with any handicap is to treat them as a regular person. Only change your tactics if you can feel that their will not understand you (like in extreme cases). Belittling them, and treating them especially will actually make them angry and frustrated.