Poor breathing is the most common musculoskeletal deficit, and it affects all ages

We have two different breathing patterns: nose-diaphragm (ND) when we are relaxed, and mouth-chest (MC) when a high heart rate dictates that we need to move air quickly.  The ND pattern draws air deep into the lungs where gas exchange occurs, and a low (10-15 bpm) breathing rate can be expected for most healthy people at rest.  The MC pattern moves air quickly but gas exchange is far less efficient which means a much higher breathing rate is needed. 
A functionally healthy individual would be expected to display diaphragmatic breathing at rest but a surprisingly high number actually mouth-chest breath instead.  A high percentage of these also over breathe (a higher than ideal breathing rate) as a compensation.  Not surprising is the fact that this problem affects individuals of all ages; young children up.  The spread of ages provides the biggest clue as to why the ameliorated pattern arises: too much sitting.  Sitting compresses the abdomen making diaphragmatic breathing difficult (though not impossible!).
The diaphragm is a muscle and, like any other muscle, 'fitness' depends on use.  A muscle that is used weakly becomes weak.  A weak diaphragm would be expected to result in the adoption of a mouth-chest breathing strategy to compensate.
A weak diaphragm is often also associated with weak patterns of anterior core stabilisation.
The good news is that this is not a difficult problem to solve: breathe through your nose down into your stomach.  It's not necessary or advisable to do so all the time; check your breathing every now and again.  The brain will eventually catch up with what you're trying to achieve and restore the correct resting ND pattern.