Without medical doctors, we would sure be lost. They know all kinds of stuff about organs and bones and brains. But when I read about some doctor suggesting seniors should lift weights, I throw up a little in my mouth. They don't call them dumbells for nothing.
As humans, we want to walk upright. But as we age, the so-called "stabilizing muscles" have tensed and flexed so much that they begin to lock up. I'm talking about your hips and waist.
As the hips begin to lock in the adducted, inward-rotated and hip-flexed position (bent over and knock-kneed), the upper and lower body will follow. Knees, feet and ankles get stiff. Shoulders, elbows, wrists and fingers get stiff. (This is starting before you turn 30).
Without stretching, the joints break down. They progressively stiffen out of alignment because some ligaments lengthen abnormally while others shorten. This wears the cartilage, causing arthritis, pain and reduced mobilization.
By the time your 55 years old, your tendons are short and brittle. Lifting weights exacerbates the problem, and can cause the muscle fibers to tear out of their boney origins or insertions.
Resistance training is just plain dumb for old people. This is why Tai Chi has become so popular at assisted living facilities. Tai Chi assists the elderly to mobilize their joints without the risk of injury. Duh,
Sources:
http://www.arthroscopy.com/sp12001.htm
(contrary to this diagram, tendons can be lengthened)