If you could only do one lifting exercise which would you pick? If you wanted the most benefit for your effort you would pick the squat. The squat works the most muscles around the most joints of any strength exercise. No matter what your age! If you can sit down and stand up from sitting you can squat. Learn to squat properly to maximize your strengty training. No matter what your age you need stronger muscles and bone density.
But you do need to learn the correct technique. Here is an excellent instructional video from Mark Rippitoe of StartingStrength.com.