What age does your muscles fully develop ?

All present @ birth: You r born with all the muscle fibers u r going to have. Biopsied they look like a wt lifters muscle just smaller. If u mean being strong and defined, this is accomplished by wt training, running etc. Start @ young & u may injure growth plates so extra caution is needed in early 2 late teens. Best a well supervised program always. Seek out an at-c/trainor to guide u & not b injured.