That's means, people needs to responsible with anything he doing, including if he decides to playing gambling even if his religion prohibits gambling. He will choose what he wants to do and must knows what the consequences of what he do so he can thinks what he must choose.
People who playing gambling and knows that gambling is prohibit in his religions will have their opinion and we can't force them to leave gambling. They will decides when they will leave gambling and while they still playing gambling, we can't do anything and only watch they playing gambling. Related to sin because of prohibition on their religion, no one will knows about the sin so that makes those people still playing gambling. But if someone decides to playing gambling, he must be careful and always use limitations.
It's because 18 years old people are already mature and they should able to distinguish between good and bad thing. That's why mostly school designed to finish at 18 years old, since they're free to choose whatever they want and should be know what they need to do next.
Most people nowadays don't care about sin, what they want is money and satisfaction because with money you can solve every problem in the world and satisfaction make you know why you need to live long.