In 2009, a large-scale study was completed with 1,644 men and women of varying ages. They performed one set each of 10 different resistance exercises, 2 or 3 days a week, for a 10 week period. On average the participants added 3 lbs of lean muscle weight and lost 4 lbs of fat weight. The 65-80 age group developed muscle at the same rate as did those in the younger age groups.
Westcott, W (1999). Fitness Professional’s Guide to Strength Training Older Adults. (Thomas R. Baechle & Wayne L. Westcott). Champaign, IL: Human Kinetics.