What are some exercise plans for seniors?

I am not sure if you mean seniors in high school, college, or senior citizens, so some of the plans will need to be amended based on the age.  

Any decent health plan focuses on two things: diet and exercise. Diet is calories in and exercise is calories out. When the input and output of calories is equal, a steady weight is maintained. When the input of calories is higher than the output, that...

I am not sure if you mean seniors in high school, college, or senior citizens, so some of the plans will need to be amended based on the age.  

Any decent health plan focuses on two things: diet and exercise. Diet is calories in and exercise is calories out. When the input and output of calories is equal, a steady weight is maintained. When the input of calories is higher than the output, that is weight gain. When there are more calories being burned than ingested, that is weight loss. A person can exercise all they want, but if the calorie intake is still higher, the person will still gain weight. Any exercise plan focuses on balance. 

Because your question focused on the exercise part, I'll focus on that as well. Exercise plans should focus on two main things: cardio and strength. Strength training could be lifting weights, yoga, plyometrics, and/or resistance training. Ideally, you should do those things 2-4 days per week. You have to include time for rest. If you don't, your muscles are constantly being torn down with no time to recover and rebuild. An elderly person should avoid plyometrics, because it's fairly high impact.  

As for cardio, running, biking, swimming, walking, etc. are all good ways to build cardiovascular health. Running and walking have the advantage of building and maintaining bone density, because they are both impact workouts. Swimming and biking are not impact training, so they are good for joint health. You should shoot for 3-5 days of working out. Vary the intensity and duration of the workouts too.