Brick and stone walls will begin to show age anytime after 20 years.  As soon as you start to see some problems, it is important to have the mortar repaired.  Without attention, the entire wall will begin to fail and eventually collapse.  Some of the signs are:

If you have concerns, it is best to call a professional to examine the structure to see if repairs are warranted.  The solution could be re-pointing, tuck-pointing, or some other process. 

Re-pointing is the replacement of any damaged mortar.  Over time the mortar joining brickwork will become brittle from age and the cycle of winter freezing and thawing that infiltrates the tiny cracks.  Replacing that mortar is essential to the safety of the structure.