For instance, tobacco products are illegal to sell to someone under the age of 21. It's not because people magically become responsible on their 21st birthday, it's an arbitrary line drawn in a way to balance personal freedom, public welfare, and the need for cigarette manufacturers to make billions of dollars.
As an outside observer looking in, online ads for weight loss products seem to be 70% scummy, 25% outright fraudulent, with maybe 5% of them selling something of value. I don't believe beauty and fashion products have a similar ratio.