As we get older our bones become more brittle as they start to lose some of the minerals (e.g. calcium) in them – this is often more common in women than men. It happens all over the body, including in the bones of the back.
This can cause back pain in a few ways:
1) nerves can get trapped (this is called sciatica), sending pain signals back up to the brain.
2) the little rubbery discs that sit between each vertebra can get worn and squashed
3) the vertebra themselves can get tiny fractures in them – this is what causes the hunched posture of some old people
4) because the body is weaker the muscles in the back may have to work a bit harder in everyday tasks like lifting and carrying, which again can cause fatigue and pain.