So, basically, the region would be gone (as if it was offline) to any teen. That is probably the only true way to prevent accidental mature content shown to teens. Except for the pervs which have that stuff in PG sims, but then it wouldn't be an accident, as that is a clear violation of the rules.
However, I (being a teen myself) do not know exactly how the main grid mainland is made up, but wouldn't this cause many sims to become not only impassable, but also render the mainland as nothing but a mesh of sparcely connected, and thus invisible, regions? If the sim bordering a PG region is Mature, the sim one region further will also not show up, because it is disconnected. How are we ever supposed to take the train, or drive your car in that? The only real method of transportation would be to teleport all over the place. Even if it just to get to a region just one sim away.
On the topic of teens using fake information to get into mature/adult parts of second life: it already happens, and probably on a pretty big scale too. The only difference is, that in that case, it is the teen who is at fault. Any claim by the teen wouldn't hold up, as he/she clearly circumvented and bypassed restrictions to get to that stuff in the first place.