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If there wasn't any age limit of 18 years and above allowed to gamble, it would have been made possible for the government to control gambling. But since someone above 18 age old is viewed as adults who are old enough to make or take decisions on their own, the government doesn't have control over their lives, and how they live it. The least the government can do is to tell their citizens the disadvantages of gambling. How to deal with it to avoid being addicted to it, that someone can't live without
This rule also depends on each country, I mean each country has its rules. My country does not legalize gambling regardless of age, so any form of gambling is prohibited by law. But "gambling is everywhere", and the government is still having a very difficult time eradicating gambling because there are many government officials (person) who support it.
Even though the ban really exists, gambling always exists. Anyone who gambles is basically prohibited in the rules, gamblers know the consequences and they certainly have to consider them wisely. If they gamble in secret, it could be that the government does not know about this activity, meaning that much of the gambling is done secretly.