Hi Mary Ann,
You aren't old, and I always say that age is just a state of mind. On my next birthday which is May 9th, I am going to be 53, and I do have a creaky right knee.
There are ways to make your knees stronger, and you should work your quadriceps muscle, which holds the patella in place. Stretching also works wonders to alleviate the stiffness. Everybody on this thread has had wonderful advice, and most of Cathe's high impact moves can be modified to low implact. I alternate step with kickboxing, and I also do strength training which is very important as one gets older. I would purchase the basic step DVD, perhaps Kickmax when you get through that. The basic step DVD also has Body Fusion. As others have said, you can do it on a four inch step. The Stretching DVD is also great.
Welcome to the boards.