Given that the human brain is still forming into your 20's, the harm can happen at any age. And given how much social media pressures impact teenagers, it doesn't even require that early of an age.
Are you suggesting that my comment is incorrect, that parents are okay to allow access to smartphones/tables/etc. for their children, because our brains can be harmed by social media at many other ages?
I believe that is what they call whataboutism, unless I misunderstood the point of your comment. When people comment at me I usually assume they're attempting to correct me.
Given that the human brain is still forming into your 20's, the harm can happen at any age. And given how much social media pressures impact teenagers, it doesn't even require that early of an age.