As a Trainer, I have worked with a multitude of people. Young and old, many cultures and many different backgrounds. The question is, can everyone be trained?
When it comes to technical skills or a computer program, there are usually set ways to perform a task.
When it comes to soft skills such as customer service, it is more of an environment, an organizational environment as well as what is inside of each person to rise to the level they are being asked to.
Are there individuals that can't or won't learn what you are teaching? In my past work, I have seen people that didn't want to conform to the ways of a company and they eventually worked their way out of the organization as they didn't fit in anymore. They found themselves waiting at the station and the train moving on.
When it comes to technical skills, it could be possible that someone just cannot retain the know how needed to operate a machine, a computer program, etc.
I believe that almost everyone can be trained. There are many pieces that have to come together and part of it is that they have to be open to learning.