By Dorene Internicola NEW YORK (Reuters) - America's ageing population is posing special challenges, fitness experts say, because it is difficult to design effective workout routines for people with such a wide range of abilities. For one 70-year-old, the goal may be to run a marathon, for another it's getting out of a chair. "If you are teaching 10-year-olds, it's perfectly reasonable to do an activity that everybody would participate in," said Dr. Wojtek Chodzko-Zajko, an expert on aging with the American College of Sports Medicine. ...