Having good posture is something that we all desire for aesthetic reasons or for health benefits. It is not until we get pain or it is brought to our attention by a loved one or a family photo that we begin to think that some action should be taken. It is never too late to address it but the longer you wait and the less you do, the harder it will be to achieve the desired results.
From the time we are born our musculoskeletal and nervous system develops in conjunction with how we move and the stimulus from the external environment. When we are adults it becomes an effort to keep good posture because the activities we perform regularly are repetitive and usually moving our joints within a narrow range and our bodies adapt to this narrow range. This narrow range of movement creates a faulty posture.
Simply said, if you want to be able to look fully over your shoulder, raise your leg to tie your shoe, or reach for something from the top shelf then you must practice those movements regularly. If you avoid the systematic practice of such wide range movements then the joints and connective tissue in your body will stiffen up and an attempt to go outside of that stiff range will be painful.
In many cases, the pain is coming from adhesions in sliding surfaces between muscles and joints, which can only be released by manual therapy, rehabilitative movement, and a change of habits.