Why is it extremely unlikely for the development of powerful lungs to occur after the adolescent period?

The size of the lung will be increased as someone growing their height. Higher height will result in a bigger lung and then higher lung capacity. Height will be increased much in puberty period because of the growth spurt but it won't go much after that.
Since there are no possibilities for achieving higher height in adolescent, the lung capacity wouldn't become higher too. 

C- a hypothesis or d- an observation