Well that's the question isn't it, agency but to what extent. It makes sense to allow teenagers to have more agency over themselves than a 10 year old, but clearly there is an argument for limiting their agency on social media platforms.
Just because there are adults that abuse their agency of the use of alcohol (alcoholism) doesn't mean we should remove restrictions on young people.
Just because there are adults with full agency who drive recklessly doesn't mean we should allow 13 year olds to drive cars.
The point is very clear to the rest of us. You inserted the word "full" where the author was not talking about full agency. It's hard to believe that you don't follow.
We are arguing over how much agency children should have. The article argues they should have more, the comment you originally responded to said practically definitionally we limit the agency of children. The obvious implied point is that it's not unreasonable to think their phone use may be part of the agency that they are denied, and that would not be a departure from how children are treated.
