Is osteoporosis a specific stage of the lifespan or a stage of life?

No, osteoporosis is a class of diseases where bones become weak and can easily break. It can happen to anyone (especially if their diet is deficient in calcium or they do not get enough sunlight to make vitamin D), but the most common form happens in menopausal women due to changes in their calcium metabolism.

Osteoporosis can also be a side effect of certain treatments for cancer, if the treatment kills or suppresses the osteoblast cells that are responsible for building bone tissue.

A metabolic disorder that stimulates the activity of the osteoclast cells that are responsible for breaking down bone tissue can also cause osteoporosis.