There are people who are old before their time. I've heard 30-somethings say "I'm too old to..." And many of us partied a little too hearty in our 20s to be much use in the office come Mondays. Then there are 100+-year-olds who run marathons and/or are dedicated to a cause. Working definitely does keep many people young, and affordability of retirement is a clear issue at present. So everyone should be judged on their merits. Age is not nearly as important as attitude. As more people are healthy at 100, then more people will want to work at that age and be capable of it. If that increases flexibility in the workplace, then that's good for everyone. If no one hires them, they'll probably become entrepreneurs in competition with those who rejected them.
As someone who works at home, I would say the most important is to be clear on your work priorities, and how success will be measured. I also would say set boundaries with friends and neighbors, as well as family, you'll be amazed what a variety of people think it's OK to impinge on your work time. But even so there are bound to be some interruptions, especially at first. Producing greatly successful results is more important than number of hours spent sitting at your computer.