Why does everything which is mailed to you require you to be "18 or older"?

Maybe most free samples include some porn? That’d be sweet.

Free samples are sent in the hopes that you’ll like the product and buy it. Assuming you are referring to common free sample items like shampoos, soaps, and other domestic items, most kids under 18 are not that target audience for such things, so it is not worth the company’s money and time to send them out. There may also be some legal ramifications but I’m not aware of those.

The first things that come to mind are targeting the audience and protecting the supplier against some legal liability.

Mail-order marketers are perpetually ravenous to add new names to their mailing lists, and there is a huge market in mailing lists themselves. When you see free samples and discounts offered, they are a clue to how much your name and address are worth to somebody. If a vendor can do anything to narrow your interests from a global range of possibilities, your value to advertisers goes way up. Multiple data points and you are a gold mine. Then they can sell your information to vendors of products of that type and also all the related products where demographics show a correlation—for example (just a made-up example, not a fact), that buyers of expensive cameras are more likely than others to also be interested in audio equipment.

The fact that you bought something in response to a certain kind of advertising—or donated to a cause, or whatever—is a data point in itself.

There is also the fact that demographics for the product may indicate the highest potential in a certain range—say, women 18–25, men 18–49—and so they want to target those groups, the ones likeliest to buy and with the highest discretionary income. It is always worth the most to marketers to narrow their target as much as possible, meaning more returns for every advertising dollar spent.

Among those potential advertisers might well be some who want to sell you over-18 products. So they would want to assure themselves that they were avoiding exposing underage customers to adult content and, again, targeting real potential buyers.

It’s for accountability purposes.

@The_Compassionate_Heretic, accountability for what?

Are you trying to order ninja stars again? Dude, they won’t really send them to you, at least not in California.