even though this industry is still largely unregulated, the word has gotten around that you need to be certified to conduct exercise classes of any stripe. You will also not be able to get professional liability insurance unless you have a certification. You will need this insurance unless you are employed by a senior living community in which case they should carry insurance for all employees.
There are many certifications to choose from, Silver Sneakers being just one of them. I would suggest that you also check out the Senior Fitness Association if you plan to focus on seniors.
I wish you good luck.