People can’t ignore their age. Making this mistake can lead to weight gain, heart problems and chronic illnesses brought by unhealthy lifestyles. The older you get, the more wear and tear you have, Shane McLean, certified personal trainer at Balance Guy Training, says. People lose muscle mass and function every year after their late 30s. Those who are physically inactive can lose as much as 3 to 5 percent per decade. This means you have to rebuild muscle to increase the metabolism, which helps regulate weight and blood sugar levels, but this is much easier said than done.