Social media firms are being threatened with new laws if they don't do more to protect children online.
He gives them until the end of April to outline action on cutting underage use, preventing cyber bullying, and promoting healthy screen time.
Google and Facebook say they share Mr Hunt's commitment to safety.
The age requirement to sign up to Facebook, Instagram, Twitter and Snapchat is 13. To use WhatsApp or to have a YouTube account, you must also be at least 13.
In his letter to the internet firms, Mr Hunt said: "I am concerned that your companies seem content with a situation where thousands of users breach your own terms and conditions on the minimum user age.
"I fear that you are collectively turning a blind eye to a whole generation of children being exposed to the harmful emotional side effects of social media prematurely.
"This is both morally wrong and deeply unfair to parents who are faced with the invidious choice of allowing children to use platforms they are too young to access or excluding them from social interaction that often the majority of their peers are engaging in."