The low back is called the lumbar spine and has 5 vertebral bodies separated by 5 pads called intervertebral discs.
As we get older the discs loose their water content either suddenly, called a herniated disc, or more gradually called a degenerative disc causing the vertebral bodies to move closer together. When this happens the adjacent vertebral bodies rub more than normal (at the facet joint) causing pain, stiffness, and weakness.
The muscles of the low back then become weak because it hurts to move and we also become stiff. So then we get a painful stiff and weak back.
We can't reverse the normal age related changes in our discs and facet joints but we can reduce our symptoms if we restore the flexibility and strength in our back.