Training the senior citizens

I taught my Intro to Microsoft Excel class today. I had 7 people attend. Among them was an elderly lady who was given a computer from her grand children. She's approaching 90 years old. This is the third time she has taken the class and I feel that she is not making progress. She always has difficulty handling the mouse and when I lecture and show by example on my projector screen, she always manages to get lost on her computer (for example, accidentally shutting down her computer when I have the class open a program through the Start menu).
She's a good soul. But at what point do I tell her that I can do longer teach her. Her mistakes interrupt the progress of the class and I sense that the younger students become impatient with her whenever problems come.

Do I need to organize a special class for the elderly at the public library?