If you read what I wrote, you'll see that I said that once you're already solid on gaze coordination, then I think 3D video that you could easily track wouldn't do harm. Those are caveats, and are probably related not just to age, but individual capabilities (as I said I'm just an anecdote).
Secondly, if you read the article, you'll see that it's titled "3D video hazardous to your health", not "3D video hazardous to your kids' health". The article also conjectures that "one can surmise that it’s also never too late to learn bad habits that could create visual problems". So, if you read the article carefully, you'll see that it is not mostly concerned with the development of children's visual pathways, but rather is interested in promoting fear for adults too.
Secondly, if you read the article, you'll see that it's titled "3D video hazardous to your health", not "3D video hazardous to your kids' health". The article also conjectures that "one can surmise that it’s also never too late to learn bad habits that could create visual problems". So, if you read the article carefully, you'll see that it is not mostly concerned with the development of children's visual pathways, but rather is interested in promoting fear for adults too.