My 60 y/o mom is on social media for hours per day. That's harmful.
My 80 y/o grandma plays Pokemon Go four hours per day, which we initially thought was good because it got her to go walking. Now we can't take her anywhere without slowing us down. It's become harmful.
My 6 y/o nephew will totally tune out the world if you let him play Angry Birds, ignoring his parents until someone physically removes the screen from his hands. That's harmful.
I hosted a field trip for pre-teen middle schoolers and in our downtime between activities there was zero social interaction because they all buried their heads in their phones (okay, I did observe one student ask another for a charger). That's harmful.
Screen time is hurting everyone and singling out kids is just old people refusing to accept that screens changed humanity at all ages. I don't think that our current old-timers lacking screens when they were kids made them any better at life. It only made them worse at using modern technology. The current crop of youngsters may miss out on face-to-face interaction, or maybe their eyes will be shaped poorly for long distance sight. But they'll be better at navigating the digital world, which seems to be where we spend a lot of our time regardless of age.