“Our lab and others have shown repeatedly that older muscles will grow and strengthen,” says Marcas Bamman, a professor of integrative biology at the University of Alabama at Birmingham. In his studies, men and women in their 60s and 70s who began supervised weight training developed muscles that were as large and strong as those of your average 40-year-old.”
Source: Can You Regain Muscle Mass After Age 60? – The New York Times
Dr. Bamman says older folks (over 60?) don’t add new muscle fibers like young’uns do. But an effective exercise program will cause hypertrophy (growth) of the existing muscle fibers. “Effectiveness” probably depends on exhausting muscle groups during weigh training.