Aging by itself is a key factor in changes that occur within the intervertebral discs. However there are studies that show the disc spaces getting higher as we age and others showing smaller disc spaces. So we don't have all the answers yet.
A recent study in Germany looked closely at the changes that occur in the spines of former elite track and field athletes. They found the smallest disc heights in marathon runners. The largest spaces occurred in jumping and throwing athletes.
At first the researchers thought this made sense. Exercises like jumping or throwing load the spine unexpectedly. This would cause positive effects on the nutrition to the disc and higher disc height. Long-distance running with its high compression loading could reduce the nutrition.
However, higher loads like long-distance runners are found in discus throwers and shot putters but they have the largest disc height. Perhaps genetic or other unknown factors are at work here. Scientists say it may be the result of complex and unpredictable interactions of multiple factors.