On a Web site use medium or boldface type against a light background. Avoid putting shades of blue and green together. Feature short page lengths and linear navigation Keep what you are offering simple. Test it on seniors. If it is too difficult try something easier.
Games are very popular with seniors Give the senior a written copy of the procedure.
Having something to follow that is familiar is a comfort and a way to double back and check for errors. If you are training at the organization's office, insist on a large monitor with large tracking ball and mouse that is not overly sensitive. Customize software to increase font and icon size. Everyone, even children, have trouble when they start. Be encouraging and kind to those learning seniors.