Why do we stop growing ?
The genetic code is coding system through which information encoded in DNA or RNA sequences are translated into proteins or hormones.
The DNA or RNA sequence will determine which protein are to made and this specific protein made will determine specific activity. This how nucleus is define as the control center of the cell as genetic material DNA resides in the nucleus that control protein synthesis.
We stop growing because of our genetic code that controls growth processes of our individual.
If bones or reproductive systems are matured then it indicates that physical growth is complete and reproductive glands increases the production of hormone estrogen that results in fusion of growth plates of the bones. It closes the growth centers of bones and thus they are not able to initiate the growth.