When strength training to gain muscular weight it is common knowledge you need to add calories to your diet to maintain the newly developed tissue. When you reach middle age things change. Ageing results in a gradual decrease in size and volume of lean muscle and its subsequent mass reduces each decade. Though part of the strategy for maintaining muscle mass in middle age is similar to when you were young, that is to habitually be active and strength train; to slow the gradual loss of muscle the nutritional approach necessary to maximize maintaining lean muscle is actually counterintuitive.
Researchers have found that caloric restriction attenuates age-related muscle loss. In aged muscle restricting calories leads to metabolic reprogramming of the pathways to derive energy. For the science based reader it means that there is a decreased dependency on glycolysis and an increased cellular dependency on oxydative phosphorylation. It is speculated that you should reduce the amount of calories you need by 8% when you reach midlife, which inturn allows you to maintain the highest amount of muscular tissue. It is also recommended by researchers that the protein you eat is high in leucine (leucine is the dietary amino acid that has the capacity to directly stimulate muscle protein synthesis) with foods such as cheese, soybeans, beef, chicken, pork, nuts, seeds, fish, seafood, and beans.
The bottom line is as you age eat less to maintain more. Get Strong and Stay Strong.