What if I'm not an adult? Can I not participate? Can I not read the question? Is it somehow inappropriate for me? Of course not. Nothing, especially on the internet, is just for adults — especially toilet technology, which is obviously used by people of all ages, and, ironically, is more freely talked about among children.
We are not all adults here. Saying that we are're adults is wrong, irrelevant, and condescending. The phrase is just fluff, which does not belong in an SE post. Given that the phrase is applied so broadly and indiscriminately, it provides no value and should be quashed. If it "isn't supposed to be taken literally", not only does it not add value, but it conveys literally no meaning.
Stack Exchange already has a tacit policy contained in its culture that the tone should always be professional on main sites. This phrase is just fluff. Remove it.