Although children can begin weight training at a younger age, they normally do not gain muscles until they reach puberty, when hormones allow them to increase muscular mass. Teenagers who lift weights and exercise aerobically cut their risk of sports injuries in half. The best time to start building muscle mass is before you reach your 20s. Research shows that adults as young as 18 can build muscle strength and size if they follow a progressive program of resistance training and eat more calories than they burn. Even teenagers have been shown to increase their muscle mass and bone density as well as decrease their risk of injury by following these guidelines. Building muscle mass early in life allows you to become stronger and larger later in life. This is especially important for people who want to play sports or work with heavy equipment. Strength training helps you manage pain after surgery, get back on your feet faster, and protect yourself from further injury. The more muscle you have, the more energy you have available. More energy means less chance of suffering a heart attack, stroke, or other disease-related complication. Exercise is also very important for keeping you healthy as you grow older. It reduces your risk of falling, developing diabetes, and dying earlier. If you aren't using your muscles, they will eventually waste away causing you to become weaker and more prone to injury.