You can fly Part 91 and Part 135 after age 65. Realistically, you probably won’t want to: normal aging and the desire to pursue personal interests might compel you to stop flying for a living around 65. That’s not to say that people don’t fly for a living well past 65, but there aren’t great numbers of them.
Welcome to the site. I've never been up there, but Action Multi in Groton, CT seems to have a good reputation. 90% of my civilian instructing here in NC is Commercial Multi Add-ons. Your profile says you're private with 850 hours. I'd say no matter what you do, study for and pass the commercial written before you try to enroll in a flight syllabus. It won't a slow, steady 40 hour marathon like your private and instrument.