The proposed measures would would apply to platforms such as TikTok, Facebook, X, Instagram, Snapchat and YouTube, as well as gaming services that allow contact between children and strangers.
Messaging apps such as WhatsApp and Signal would be exempt.
"It is clear to me a full ban is the right choice," Starmer told reporters on Monday, arguing that the measures would help protect children and improve their wellbeing.
"It will make a huge difference; it will make our children safer; it will make our children happier; it will give them more time, more security, more freedom to grow up and more opportunity," he said.
The British government is also considering additional measures, including overnight curfews and restrictions on infinite scrolling for users under 18, the Reuters news agency reported.
Australia in December became the first country to introduce a social media ban for children. Britain would go further by extending controls to gaming platforms and other online services, according to Reuters.
Some experts questioned whether a blanket ban would be effective and argued that enforcing the restrictions would be challenging.
Social media companies said a ban could drive young people toward less regulated platforms that lack the child-safety protections introduced by major firms in response to tightening regulations.
Starmer said the ban could take effect next spring to "give kids their childhood back."
Services designed primarily for children and education, including YouTube Kids, Lego Play and Google Classroom, would not be affected by the ban.
A host of other countries, including Poland, are also considering tighter regulation of children's access to social media amid growing concerns about the impact of online platforms on young people's health and safety.
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Source: Reuters, PAP