"We are all adults here" seems to be a clickbait added to titles of hygiene questions. One such question was asked in 2011, another was asked in 2013, and starting in May 2016 it seems to have started to become a meme.
I'm not the only one to have noticed this trend:
28⇧ Do we need an all-adults-here tag yet? – Henning Makholm yesterday
Anyone can read Stack Exchange questions and answers. The CC BY-SA license makes it effectively impossible to discriminate against viewers by age.
Furthermore, the Terms of Service prohibit users under age 13 from signing up, so younger readers can't really say "Hey, I'm not an adult!"
Is there any value to the "As we are all adults here" phrase? Can we quash it?