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.
To some third parties, it may look like you have no neck and that you became shorter than before. While it may look silly, there is nothing more effective in preventing yourself getting rocked or hurt other than avoiding or blocking a strike.
One example of someone who frequently has his chin tucked and stiffened for punches is Gennady Golovkin. He is also frequently seen doing neck exercises for his chin to be able to resist punches. While you cannot train your brain, you can lessen the severity that your head moves when the chin takes a strike.
The chin represents an easy lever that can be exploited to push something with little force. Think of the wheel or using a long solid object like a thick piece of wood to lift something very heavy. This same concept is applied to the chin in that it takes very little force to move your head if you apply force to that spot.