Coaches have a standard life time of 25 years. However coaches that are more than 15-18 years old, if found to be in a reasonably good condition, are repaired (or rehabilitated) at designated workshops to extend their lifetime by a few more years.
Not all coaches undergo this, but if a coach has been maintained in a relatively good condition, its life time can be extended for a short time by doing the necessary replacement of parts and repairs.