The problem Apple will run into with iPhone, and to be clear this isn't actually a problem at all -- quite the opposite, is that the iPhone is immediately accessible to everyone. The iPhone's target market is far larger than the classic smartphone target market, because anyone can use and appreciate the iPhone.
The problem with the teen market is: You can't convince a teenager that her grandfather's phone is cool.
This should be the Google Glass target market, and most likely will be the only large demographic with reasonable saturation: Google Glass encourages sharing too much, something most teens love. Google Glass alienates people who don't appreciate the sharing too much trend, another thing most teens love.
The only foreseeable problem with this strategy: Google really needs to get the entry price for Glass down to iPod touch prices, or at the very least those of iPad.
*note: Glass also has the potential to be huge in Japan, and possibly also (South) Korea. But teens everywhere will be Glass' biggest consumer/user base.