Though I've always wondered does the whole body grow...
Basically mate it is all down to genetics it is a natural process that is "programmed" to stop at whatever is laid down in your genes.
Just a little bit of trivia for you - did you know that you as an individual your current body only ever ten years old?? - everything regenerates (at different rates of course ie liver every four years, red blood cells every 120 days etc etc ) so that the body is only ever that age. Of course I am not including - in my mind at least those cells which do not replicate so easily if at all ie nerve cells
Hence aging - the cells never replicating exactly as they were before.