In your 30s, you might not be thinking about muscle loss yet, but Alexon suggests you should start. "Once you hit your 30s, we start to see age-related muscle loss a little," she says, adding that, without the help of diet and exercise, your muscle mass will continue to decline slowly but steadily from that point on. So what should you do? Alexon suggests a dual approach: "lean protein spaced throughout the day [and] resistance activity, not just cardio."