It's often said in a joking manner, but all jokes aside, back pain is NOT a side effect of aging. Examples of true aging side effects include a decrease in speed of digestion, loss of vision, and playing BINGO. Lower back pain is often a side effect of having weak lower back muscles.
The picture above is a profile view from the right side of the spine. The thick, marshmallow-shaped bones on the right side are the vertebral bodies. Vertebrae have wing-like structures on both sides called transverse processes (the right side is pointing at the viewer). The vertical yellow tube is the spinal cord, which travels through a hole in the middle of all of the vertebrae (only seen from a view above). The spinal cord has roots that branch out between the vetrebrae's body and "wings". Lower back pain results as a compression of the spinal cord or its roots. In most cases, it is the vertebrae that collapse on each other, narrowing the space between where the nerve passes through.
As you see in the picture above, the red line indicates the compressed nerve. Notice that the space between the top two vertebrae is a little greater than the space where the compression is. Strong lower back muscles (especially a muscle called the multifidus) maintain a healthy length of the spine where there is enough space for the nerve to pass through without any impingement.
Unfortunately, the most popular treatments used today don't offer long term solutions. Examples of these are pain medication, weight lifting belts, and inversion tables.
However, strength training does offer a chronic solution by increasing the space for nerves roots to lie in. It strengthens the multifidus and other lower back muscles, making them more capable of handling those tough tasks. Finally, training the lower back brings a greater blood flow into the area. With increased blood flow comes more nutrients and greater removal of undesired substances/tissue.
If you have back pain, consider a lower back strengthening routine. Even once set per week has relieved pain for many of my clients.