So as you get older there is a loss of muscle, sarcopenia. That can be reduced by exercise, but still the older body is not as good at synthesising muscle. Also as you get older people tend to eat less protein. You can give them protein supplements....but people don't always like to take them. This study gave them Leucine supplements instead: Leucine supplementation chronically improves muscle protein synthesis in older adults consuming the RDA for protein. It looks promising.
.......interesting things about fitness, strength, diet and performance.