By the time a buck reaches 3 1/2 years of age it has roughly 97% of its skeletal mass. So for the first 3 1/2 years of its life, a buck is putting the majority of its nutritional intake towards building its skeleton. After 3 1/2 years of age a bucks nutritional intake can then shift toward building muscle and fat reserves as well as pumping more into its antlers. Ever notice how a bucks' body size and antlers take a noticable jump after 3 1/2 years. I've always read that a buck is not fully matured until it reaches 5 1/2 years of age.