While many probably thought a rule like this was already in place, it’s actually a new guideline. Disney states that no particular incident triggered the new rule.
To enforce the rule, Cast Members manning entry points will monitor guests, and engage them in conversation if they suspect the person in question is under 14. If so, the parents or guardians of the child will be contacted to accompany the child into the park.
The change takes place for both Walt Disney World and Disneyland Resorts. There is no word on age restriction at other Disney parks around the world.