We already have forum rules (in particular 7 and 8) that say to engage in respectful discussion. The 18+ rule means you also have to literally be an adult, so that nobody sues us for discussing e.g. LGBT topics.
If 18+ has some coded meaning, like “be mature,” rather than what it says on the tin, then that kind of undermines the integrity of all the other rules. Now I have to second guess if the other words have some double meaning, too.