A cynical hypothesis to explain why smut (sex, violence, drugs, and so on) are characterized as "adult": because describing smut as inappropriate for children comforts adults about the dangers of smut without requiring adults themselves to forgo such guilty pleasures. Note that the experiments I've run so far have been unsupportive.
Sex, violence, drugs, profanity, and gambling—in a word, smut. People know to a lesser or greater degree that we'd be better off without smut, but they also enjoy it—or at least, they enjoy media depictions of it. And so whereas children are expected or even legally required to avoid such influences, adults are usually permitted to indulge in them. Now, how exactly does the first discrepancy give rise to the second? The standard argument is that children can't handle smut whereas adults can. But this argument is supported neither by logic nor by empirical evidence. While there is reason to believe that children can be harmed more, at least in some cases, adults are hardly immune to these dangers. It's naive to think that adults won't be swayed by media violence or that adults can make good decisions about drug use. There are even cases where children may be safer than adults. For example, children's attitudes towards smoking seem to grow more positive with age (O'Connor, Fite, Nowlin, & Colder, 2007). Never mind that because the effects of smut aren't guaranteed (not everybody who smokes dies of it), individual differences in susceptibility to smut are necessarily greater than age-group differences.
I propose an explanation of the impulse to protect children but not adults in terms of vicarious restraint. See, the people who get to decide what's safe for which age groups are without exception adults. When they, for example, characterize pornography as "adult content", they're placing no restrictions on their own behavior. Yet they still get to experience the virtuous feeling of preventing smut from doing harm. Vicarious restraint comforts adults about their (ultimately legitimate) fears without making them control themselves or actually reducing the danger. Although it isn't quite hypocritical, this perverse sort of double-dipping is a close cousin of how some famous televangelists have condemned licentiousness while patronizing whores. A better analogy is double sexual standards for men and women.
You can argue about whether vicarious restraint is motivated more by the desire to actually do good or by the desire to appear to do good. In most cases, I think, good intentions play a large role. In other cases (Henriksen, Dauphinee, Wang, & Fortmann, 2006), I'm more cynical.
That vicarious restraint can protect us is an illusion we cherish at our own peril. Children don't respect double standards any more than adults do. And given how many adults drink or gamble their lives away, grown-ups are at least as deserving of protection.
Henriksen, L., Dauphinee, A. L., Wang, Y., & Fortmann, S. P. (2006). Industry sponsored anti-smoking ads and adolescent reactance: Test of a boomerang effect. Tobacco Control, 15(1), 13–18. doi:10.1136/tc.2003.006361
O'Connor, R. M., Fite, P. J., Nowlin, P. R., & Colder, C. R. (2007). Children's beliefs about substance use: An examination of age differences in implicit and explicit cognitive precursors of substance use initiation. Psychology of Addictive Behaviors, 21(4), 525–533. doi:10.1037/0893-164X.21.4.525