Cocker Spaniels can be considered to be fully grown when they are 12 to 18 months old. Based on their survival needs, instincts, and habits learned, they grow up fairly quickly. If you are willing to train your Cocker Spaniel puppy, it is advisable to do so before they reach their maturity. It would help you inculcate specific habits that are essential for a socially healthy and safe living. Once they reach their age of maturity, they become less adaptable and trainable by owners as well as dedicated trainers.