Muscle loss means your metabolism will slow down. Slower metabolism means the body is burning less calories which can cause you to gain weight.
Muscle is an active tissue.
A pound of muscle burns 30 to 50 calories per day just to maintain itself. Add three pounds of muscle and burn 630 to 1,050 extra calories per week. A pound of fat only burns 3 calories per day. So you can see by lifting weights and gaining muscle you will burn more calories every day. Strength training can increase your muscle mass and this can happen at any age.