If you have problems with your knees and find it difficult to stand for long periods, how about doing Coach Nicole's seated cardio workouts ? She has a couple posted. These are exercises that can be done while sitting in a chair. www.sparkpeople.com/resource/videos-detail
There are more chair workouts posted on YOUTUBE. Just search on chair workouts or chair aerobics. Watch them first before trying them. That will give you an idea of whether or not they would be something you'd like to do.
Can you walk without any pain ? If you can walk, take a daily walk. There are members who will tell you how they lost 100+ pounds just by walking and watching what they ate. If you can't walk for 30 minutes, then walk for 10 minutes. You can break your workouts up into shorter segments and still get all the benefits. three 10 minute walks spaced out through the day still adds up to a 30 minute walk.
And as others have noted, when it comes to weight loss, what matters most is what we eat. Good nutrition is what takes the weight off and keeps it off. Exercise is what keeps our bodies fit. So, if you've been sedentary for a very long time, why not start with a daily walk ? As your strength and endurance increases with time, you can add more later.
You are not too old to start an exercise program, but you should slowly ease into a routine.