That sounds too long to me. Since later in elementary school (3rd grade), smartphone (and tablet I had at the time) have been really useful tools for me. Sure, I definitely wasted plenty of time on Minecraft videos, but it's not any worse than TV. It helped me learn a lot of what I know now. Without that, I'd have problems getting into any high school. For example, it helped me fly through chemistry and physics like a breeze. Also I learned English, which has unlocked me access to even more info. I wouldn't have been able to pass 7th grade without studying for history, again on my phone.
But of course, there was less crap than now. I am happy I got to grow up with YouTube channels like ExplainingComputers, ElectroBOOM, LGR, Scott Manley, Techmoan, The 8-bit guy, Tom Scott, Veritasium, Ted-Ed, and others I don't remember.
But even before I had smartphone, I could already watch youtube on my Sony Ericsson W200i. That was something. It ran at like 5fps in SD quality, but it worked. And 50MB was basically infinite data.