Everyone has a chin, except for some rare individuals. The chin is an interesting spot. You can consider it a major weak spot in the human body, as it provides good leverage for moving your head back and forth. Knock outs are caused by brain trauma, which can easily result from rocking the head back and forth.
A hard strike to the chin can do exactly that, and it does not take much force for a precise strike to do so. So yes, a young teen or an old man that is decently heavy can knock someone out with a flush hit to the chin or even the jaw, despite those individuals generally being viewed as weak. However, one can prevent this by the simple act of tucking in your chin. It can look a bit strange but it is very effective.