Yeah, that would get automated and fail so often that they'd drop it pretty quick. Do you know nothing about how these things work?That's because your measurement device is broken, because you live with under 18s and constant exposure to sub-18 dickery has acclimated you.
As for the rest, you incorporate it into the game. Remember, I'm not just talking about blizzard here. You can have a blizzard account without having any games, after all, and being a kid on your parent's account doesn't prevent you from playing a game (not even overwatch). But to access the separate 18+ queue would require additional verification, and if the kid gets in and plays there, and you get reported and the mod reviews the log and agrees, you're kicked out of the 18+ queue fuh-rev-vur.
Also, I feel like actually knowing teenagers (and I'm not talking about my son who is a huge outlier) gives me more perspective in this case than someone who is already wearing his grumpy old man pants.