I would rather not teach anyone but if they insist, you inform them about the implications and the addiction problem.
I would also advise against teaching gambling to under age people . I believe they can be easily affected by gambling problems.
You just tell them to be responsible when they gamble. It's not your burden anymore if they became addicted lately as long as you reminded them the implications of gambling. At least having that kind of condition on whom you are going to teach or convince about gambling is also good. Be a good example to the young ones but in our generation, kids are even more intense when it comes to gambling. To the gamblers at the right age, telling them to enjoy gambling is fine IMO.