Make two more layers, 2nd one up, make it black, then disable it so that the base texture map is seen.  Go to the 3rd layer, locate the bottom of the feet and draw white lines to represent the wrinkles, locate the palms of the hands and do the same.  When you  are finished, enable the black (2nd layer), save it as a .jpg name it wrinkles.  Save the working copy, the one with layers, in a format that saves layers, name it wrinkles too.