It's a difficult one. For instance, a 16 year old who's got pregnant by accident and decided to keep the pregnancy might turn to MN for help and advice, and that would be perfectly understandable and legitimate (though I'd hope MNHQ would keep a close eye on the thread in case of online bullying - which there would be: the "teen mums scrounging for benefits" trolls would be on it like flies round a jampot).
On the other hand, a 16 year old posting for, say, sex tips - pull that thread ASAP.
There's going to be a whole middle ground in between which would be very difficult, e.g. teen in abusive relationship asking for advice (again, could attract creeps, but where else are they going to go); teen asking for advice on binding (I view that as a form of self-harm and would hope MN would pull the thread, but you don't know...)
I feel very sorry for the MN mods on this one - it must be a nightmare. On the whole I think they do pretty well - they tend to pull threads which need to be pulled. Which is why reporting ones which seem like the wrong side of the line (e.g. a troubled teen asking for advice in what actually looks like a form of self-harm type of way - kind of inviting harsh replies so she can say "look, the Karens don't understand and they're meeeaaan") is important. MN can't make the judgement call if they don't know the thread is there.