The key figure from a new US "teens and technology" survey by the Pew Internet Project is this: 50% of teens who own smartphones primarily access the internet that way. According to Pew (the data are from Q3 2012) 78% of US teens overall own cell phones and nearly half (47%) of them own smartphones.
I suspect if the data were from 2013, smartphone penetration would have easily crossed 50% because of Q4 holiday gifts. Among all teens (including those who don't own a cell phone), 37% own smartphones.
Part of the reason that US teens may rely more heavily on their mobile phones (and tablets) for internet access is that some do not own PCs or share PCs with their families. Thus mobile devices are more private and personal because they're not shared. Regardless teens' orientation to the internet is more mobile than their parents'.
The survey also found that 23% of US teens owned tablets (compared with 25% of US adults).
As these teens "grow up" it will be interesting to see if they adopt a more "balanced approach" and access the internet equally from PCs, tablets and smartphones. I suspect their bias will remain toward mobile devices, with tablets taking the place of PCs for non-smartphone access.
An October 2012 survey (n=7,700 teens) by financial firm Piper Jaffray found that Apple held an advantage among US teens: