I tend to agree to an extent that children below the stated age should start learning the fundamentals about how to read and write. For obvious reasons, it's better for them to start getting used to the basics, because once they are groomed in these areas they can start early in developing their cognitve abilities. As it is often said kids learn faster, and thus whatever they are feed with, they are bound not to forget them in a hurry.
Although they might not be learning bigger things at this age, but with the elementary knowledge they are receiving, they can start building on it as time progresses. It won't be wise for parents to allow their kids to be influenced with all kinds of negative stuffs, as it's often experienced when they start to insult immediately they start to talk, but instead they can channel this energy in knowing how to read and write.