Many seniors feel that weight training is too strenuous, but certain weight-lifting exercises are actually an excellent low-impact way to build muscle and improve overall health. The key is to start with lighter weights, or even do the moves with no weights, and increase the amount that you're lifting over time as you improve your strength.
You'll want to do 30 minutes of strength training for each muscle group twice a week, taking at least one day off in between working the same group. For example, if you do upper-body exercises on a Monday, you'd want to wait until Wednesday at the earliest before doing upper body again. If you can, take a few sessions with a personal trainer to learn some good upper- and lower-body exercises and get tips on maintaining good form. Once you have the hang of it, you can work out on your own. If a trainer isn't in your budget, check your local library for books or magazines on low-impact weight training exercises for seniors.
Like with stretching or any other exercise, the rule with weight lifting is "no pain is good pain." If an exercise causes you pain, back off and try a lighter weight. If it continues to hurt, stop that particular strength exercise until you can talk to your doctor. It's better to be cautious than to push too hard and risk injury.