DEAR MAYO CLINIC: My grandmother lives with me, and she is always nagging me to “stand up straight” and “put your shoulders back.” She says good posture will keep me healthy as I age. But I’m only 17.

As a child, you might remember your parents reminding you to sit up straight and not to slouch. What might have been annoying then is actually pretty good advice now. Here’s why good posture is so ...