It doesn’t get old but it can change over time.
Swimming pool sand is a particular size.
It’s not just any sand.
The changes that occur will depend on what goes thru the filter.
Generally, filter sand will last for a few years.
Poor filter maintenance and water chemistry will diminish it’s effectiveness.
Like the river rocks pictured above the filter sand will become more smooth over time.
Body oils and lotions also impact the filters effectiveness.
Cycles of clearing up algae and other organic matter do as well.
When you backwash the filter it only washes away what breaks loose from the sand at that time.
The conditions mentioned above will cause a gummy effect promoting the sand to congeal.
High pH could cause scaling, low pH will make the sand erode more quickly.
Regular attention to water chemistry and overall swimming pool care will make all of the swimming pool equipment last longer.
Backwashing should be done at 7-10 psi above start up pressure.
Start up pressure is the initial reading after a fresh backwash.