This is an important question and as you might expect statistics are very difficult to ascertain due to several factors. First, we have an enormous population and there are different definitions of what it means to be mentally and physically disabled. How you define things makes a huge difference. Hence, this answer will speak from a general point of view.
According to the University of Buffalo there are about 43 million disabled people in America. However, they admit that other studies show that there are more people who are disabled. They also give a breakdown, which shows that the greatest number of disabled people are senior citizens (as one might expect). They also break it down to those who are congenitally disabled and those who became disabled later on in life. As you can see, there are a lot of variables that need to be examined.
When it comes to mental health, there are many variables as well. According to the surgeon general, about 19% of our population have some sort of diagnosable mental disorder. Some of these are very mild and treatable, and some of them are very serious. Some studies show that mental disorders are on the rise.