Most senior citizens have access to computers or use computers regularly. The vast majority have cell phones and are familiar with technology. Learning to use electronic voting machines isn't difficult, and after learning a little about the machines, most senior citizens should have no difficulty in learning to use them.
No, Senior Citizens do not feel comfortable using electronic voting machines for the same reason that senior citizens tend not to be comfortable with any new technology; a fear of change. The Greatest Generation has always had trouble adapting to new technology, so the newer electronic voting machines are as much a problem for them as are tablets and smartphones.