Callus often appears to be hard and yellow and can become uncomfortable and painful if you leave it alone.  
There may be excessive force to the area of skin involved, which could be caused by you wearing tight fitting shoes, abnormality of the bony structure of your foot, how you walk or other problems that affect your gait.

 

Areas of Callus on your feet can appear at any age, but it is unusual for children to have them.
It is best to treat your Callus before it starts to cause you pain or discomfort.