If you want to teach someone how bad gambling can lead to - show him examples of players who have lost everything in life because of the game.
Yes, this game of probability is very hard to deal when you are addicted to it. You are uncertain about the output of your actions. One thing that can help you gain some big fortune is experience and not all these guys who love to gamble are experienced. But, it is not a bad idea to ask people to enjoy gambling as long as they do not hurt themselves by playing bad and in a way that can lead to financial loss.
Teenager are easily to influence, they should be guided by the right gambler or person so they can be prevented. Lets not encourage everyone to gamble, we know its risky.
Under age people never get introduced into gambling because gambling may ruin their life itself. You cannot expect a teenager to act responsibility. After lots of social medias, I do see many young generations are not entering into gambling now a days. I guess that is a good thing for all of us.