Aging of MLCC Class II dielectrics is normal phenomenon , and it is simply a decrease in capacitance over time due to crystalline changes that occur in all Class II dielectrics (X7R, X5R and Y5V). This is caused by the relaxation or realignment of the electrical dipoles within the capacitor. This phenomenon is affected by time, temperature, voltage. 

For details please refer to the following technical literature.  Please test the capacitance of the capacitors in accordance with the de-aging step stated.