For one thing, it's fairly uncommon for children to purchase operating systems. As long as there is one major operating system with age verification, parents (or teachers) who want software restrictions on their children can simply provide that one. The existence of operating systems without age verification does not actually create a problem as long as the parents are at least somewhat aware of what is installed at device level on their child's computer, which is an awful lot easier than policing every single webpage the kid visits.