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Op said underage gambling and not gambling addiction. I am being rational in thinking. There are different status of underage; some are earning on thier own, many are dependent on thier parents and some do not have parents. If an underage has no parents or guider, then finds a way to earn from gambling as well as other things, as much as he can make good judgement of his own, he should be allowed. As long as we do not provide for him as an alternative, he should be allowed. You should only be able to control whom you cater for.
That's what's going to happen after underage gambling, they'll get addicted to it, especially if this gambling becomes a habit in their formative years, they're going to have a hard time removing that habit when they grow old, you're not rational, you're tolerating underage gambling by guising it as a means for a children to be independent.
For the hate of the addiction, underage gambling should be discouraged. I didn't consider the extreme of the scenario. Human character of adaptation shouldn't be undermined. Whatever a child does constantly from childhood to adulthood is automatically part of his lifestyle (addiction). If not corrected, it will turn generational.
But what then can one do to prevent this? Once an underage has access to Internet, he will gamble without the consent of the parents and government. I think it cannot be moderated.