PCs will never die. They are productivity machines and something of that form will always be needed in order for work to be done. Teens don't need to work, they play.
But what I can see is the PC moving to the phone, such that a monitor and workstation accessories can be plugged into the mobile device.
I believe this is what most people who argue that the a mobile device will become peoples primary computer mean.
And you can already do this. Pretty much all Android phones and tablets supports bluetooth or USB keyboards, and many supports wired or wireless HDMI out.
Even that is not enough. The raw power advantage of the PC will probably continue for some time. There is no way to cram the amount of CPU/GPU power plus RAM/storage capacity that exists in a PC into a slate style device, whether smartphone or tablet sized. That power may not be useful to a great majority of the population, but it is useful to some. And it will probably be decades before that power differential stops being relevant. Though it's hard to imagine how increases in the power of small form factor, low power devices won't also translate into even greater increases in the power of larger form factor, higher power devices.
But what I can see is the PC moving to the phone, such that a monitor and workstation accessories can be plugged into the mobile device.