Experts are recognizing that older folks can benefit from high impact exercises.
The explanation is that people who exercise tend to have less joint-related problems than others. See "Ask Well" column by Gretchen Reynolds, NYTimes, January 14, 2014.
Consider the rowing alternative. You get the full range use of joints, muscle demands from easy to strenuous, and yet you have no impact problems, no chance of slipping on ice or twisting an ankle on uneven pavement.
When you row, you have the benefits of both aerobic work and resistance training. The amount of resistance you experience will correspond to the amount of pressure you apply with your legs. It is that simple. Go for a long, aerobic row. Or do some short bursts of higher power at greater resistance. Or do some of both for a combined workout. All without getting off the machine!