Simplify the slides, splitting the information over two slides if necessary. This is both to account for the larger text sizes you will have to use and because you have to be careful not to overload the students with too much information. If possible, do not exceed three bullet points on each slide. Also do not use Excel sheets over 4 by 4 cells in your presentation as it will be unreadable by those in the back.
Increase the text size on all your text boxes. Increase the size of any text in Excel sheets to make them readable as well. If you are using a picture that has readable text, then you may need to zoom in to ensure that the text is readable.
Format your slides in a more formal manner. Do not use off-kilter orientations or animations. Given that these are older adults, they have a life outside of this classroom, so do not treat their investment of time in your class lightly. They want to know that this class is being taken seriously by the teacher, so do so by making your presentation in a more serious tone.
Email copies of the presentation out beforehand, preferably several days before. This is to allow those students who may not learn at the same speed as others to review the presentation beforehand as well as after. The more enterprising students may email you with questions about the presentation. Take these questions seriously as there will be others who have the same questions but were not proactive enough to contact you with them.