At the time of birth our skeletal system is made up of two kinds of tissues. One of these is bone. At this time our bone are made up of cartilage which is a tissue.
The tip of our nose and stiff part of ears are made of cartilage. As we grow cartilage is replaced by bone forming cells and bones are formed. The bones grow thinker when layers of new cells are formed around bones.
A growth plate is a thin layer of cartilage between the shaft and knob. This growth plate continues making new cells and in this way
bones become longer. New cells are made in the growth plate near the knob and because of this shaft grow longer. When we reach our twenties bone tissue replace the cells in growth plate. After this we will not be taller as our bones will not grow any more.