Osteoporosis results not only from primary aging but also from too much calcium much exercise little exercise few pregnancies

Osteoporosis can also result from too little exercise. 

A sedentary lifestyle puts people in greater risk for osteoporosis. Exercise helps with bone development. The lack of it will encourage loss of bone mass putting one at greater risk of losing calcium. In addition, exercise also helps with muscle strength which can reduce the risk of falling down. 

Plants cells all have a cell wall composed of cellulose.