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Note that while monetizing those under 18 runs the risk of charge backs, those between the age of 18 and 25 are still in the process of brain development and are considered legal adults. It seems unlikely that anyone in this age range, having been anointed with adulthood, is going to appeal to a credit card company for relief by saying they are still developmentally immature. Thus this group is a vulnerable population with no legal protection, making them the ideal target audience for these methods. Not coincidentally, this age range of consumer is also highly desired by credit card companies.
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To maximize the efficacy of a coercive monetization model, you must use a premium currency, ideally with the ability to purchase said currency in-app. Making the consumer exit the game to make a purchase gives the target's brain more time to figure out what you are up to, lowering your chances of a sale. If you can set up your game to allow “one button conversion�, such as in many iOS games, then obviously this is ideal. The same effect is seen in real world retail stores where people buying goods with cash tend to spend less than those buying with credit cards, due to the layering effect.