I don’t see a downside to having “middle school student”*, “HS student”, “college student” and “adult” as categories. I used to think it was clear from the number of posts whether someone was a student or an adult (parent, professional, professor…<- not meant to be exhaustive :p) But in fact one needs to frequent the forums for a while before one becomes aware of posting history and what it means, and have good critical reading skills to decipher who is a kid and who isn’t. SkiEurope, a moderator, pointed out once on a thread when I was making the point number of posts and posting history is pretty indicative: in reality, the number means little. People who frequent the forum recognize each other… but HS kids who come intermittently need to have an inkling as to the validity of the claims.
If a 17-year old posts, it’s different from a parent, even for HS kids. HS kids may feel free to disregard adults’ advice (plenty do!) and prefer teenagers’ wisdom. Or vice-versa! 
And adults take age into account when they respond (ie., the same foolish advice won’t receive the same response depending on whether the poster’s 16, 21, or 50 :p).
- important in the HS forum, where MS students ask about freshman year in HS and schedules - it's good to know your master informator about all things HS is really in 8th grade and has yet to set foot in a HS :D. Fortunately many 8th graders say their grade right away.