The "C, K, G" sounds are typically mastered by age 3, but frequent substitutions that present are /t, d/ (dut for duck, dod for dog).  "C, K, G"  are made with the back of your tongue up at the back of your mouth.  The "C" sound is typically pronounced the same as the "K" sound (cat = KAT). The "C" in "Cake" is the same sound at the beginning and the end of the word. 

These sounds are all produced in the same position. "C/K" = Voice OFF.  "G" = Voice ON.