I can see the intention of a teen.SE site and it makes sense to me.
There are teen topics which would be served just as well on this site, it's just that you don't see them here because, frankly, we have way too few teen questions and an over-abundance of baby questions. That just shows how the site came to be, but it can grow to include a lot more -- teen topics, for instance. Getting input from relatively unbiased parents is probably also worth a lot to teens.
I imagine there are also teen topics that have practically no overlap with this parenting site, or might even be off-topic here. For these, a separate teen.SE would make sense.
The four examples currently in the teen proposal seem to be valid on parenting.SE though. Granted, it's going to be hard to convince teenagers that a site called parenting is a cool place to post tricky questions!
The question I can't readily answer is how much overlap these two sites would have. Overlap is a problem because it becomes a grey area where questions might not get the best possible treatment (or exposure) because some folks believe "the other site" would be the better venue. This grey area is difficult to manage, for moderators and no less for users.