One of my early experiences was in an NHS Psychiatric hospital, as a Physiotherapy assistant.
I worked with patients aged 16-96 of both sexes who had a variety of mental health issues.
Whilst often challenging, it was immensely rewarding to discover that exercise was well received and could help break down some communication and relationship barriers.
This was an interesting time as I learned a lot about mental health issues and developed a range of skills, many of which were relevant for my future career.
I feel privileged to have gained an insight into the world of mental health and now promote exercise as one of the most relevant forms of activity for people suffering with this type of condition regardless of age, race, gender, disability, etc.
Most people will have lived an active life before they begin to develop mental problems and if they move into a care environment, their new life will almost inevitably become fairly sedentary.
This can often lead to a spiraling down effect producing low self-esteem, mood swings etc.
The sensible and careful introduction of safe, suitable exercise on a regular basis can make a big impact on a person and also on their surrounding environment:

• The person will find they will have more energy, their spirits will begin to lift and they may begin to feel able to take an interest in their surroundings and their neighbours.
  
• The atmosphere in a care environment can feel uplifted, patients may communicate more with each other and their carers, and an element of fun will be introduced in an unusual and unexpected manner.
  

It is very important that an instructor has a working knowledge of which medical and physical conditions are present within each group and (more importantly) the possible implications of those conditions on the exercises they are delivering.
With this in mind, only a suitably qualified exercise professional or a carer who has received the appropriate training for this type of activity is the correct person to deliver regular exercise to any group of vulnerable and less able people.
It is essential for instructors to have a range of motivational techniques ready to encourage participation.
One very important point when introducing exercise to anyone, whether they have medical and/or mental disabilities, is to remember that they still they have their Freedom of Choice and may choose not to join in! This is not necessarily a reflection on the instructor but simply them saying, "not today thank you" and this must be respected.
To inflict exercise on an unwilling participant is a form of abuse and is to be avoided at all costs.
However the benefits of exercise are many and varied and it is an activity that should be available to everyone.