I recently attended a yoga class near my home. I suffer from a few dodgy joints; my knees have been poor for a few years, my lower back is a bit naff and my shoulders hang forward so, as a result, they are not very flexible. The root of all of that, of all of my dodgy joints and issues, is poor posture (and some long standing medical problem in the knees.)
The yoga instructor noticed straight away; telling me sit up straight. I could not. It is not so much about stretching your core upwards, which is what I thought. It is also about pulling your shoulder blades together, pulling the shoulders back and trying to maintain that upwards hold. It is difficult, especially as someone who has had poor posture for years. But I can tell you that the effort is worth it; every day working your shoulder blades for even a short while can help your posture and your core.