Instagram will require all users to enter their birthdate to log in, even those who weren't asked to give a date if they joined before 2019, as part of the app's new child safety measures.
Users will be able to dismiss the prompt initially, although Instagram will blur content that is marked as sensitive and potentially unsuitable for under-18s, but parent company Facebook will ultimately require a birthday to let users continue using the app.
Last month, the company started to default new accounts belonging to people who give their age as under 16 into a private setting.
It has also restricted the ability for advertisers to target under-18s after criticism of its actions on child protection.
The company is introducing the measures ahead of the UK beginning to enforce its Age Appropriate Design Code, which makes several demands of social media companies and will come into force on 2 September.
The code, which is regulated by the Information Commissioner's Office, "is a set of 15 flexible standards" designed to make children safe online, focusing on minimising the data that they share with companies and strangers.
Earlier this year, Facebook introduced anti-grooming safety measures that prevented adult users from sending direct messages to teenagers unless they follow them.
However, the company was basing its ability to identify children on the age they were providing themselves, which prompted some criticism because, as the company acknowledged, "young people can lie about their date of birth".
"We want to do more to stop this from happening, but verifying people's age online is complex and something many in our industry are grappling with," the company said at the time.
"To address this challenge, we're developing new artificial intelligence and machine learning technology to help us keep teens safer and apply new age-appropriate features."
This technology is publicly stated to include checking to see if users' friends were wishing them happy birthday for an age which they hadn't registered themselves as.
It's likely, however, that Facebook won't publicise other signals which it uses to detect children.
"In the future, if someone tells us they're above a certain age, and our technology tells us otherwise, we'll show them a menu of options to verify their age. This work is still in the early stages, and we look forward to sharing more soon," the company explained.