I had to see an eye doctor at the hospital when I was ~7 and I got to do some exercises, but I never learned to cross my eyes, and then it was like it probably wasn't very important since I did not have to go to the doctor again and no one mentioned it so I just went on with my life and it seems overall like not being able to cross my eyes is not a huge problem. But I guess it may be connected to my complete inability to see 3D effects or figure out how to see anything in the images in the article.