Adults and under-18s can't message each other, in either direction.
Under 18 — you can message people within a year of your own age. Because EQUALS is 13+ and adults are off limits, that means a 13 year old can message 13 and 14 year olds, and a 17 year old can message 16 and 17 year olds.
Under 16 — you can also only accept message requests from people you've added as a friend and who have added you back.
18 and over — none of these restrictions apply.
Everyone verifies their age when they sign up, which is how we keep those groups apart.