Instagram will ask new members to provide their date of birth when setting up an account, as it plans more "age-appropriate experiences".
The photo-sharing network already requires people to be at least 13 years old to create an account.
Asking for a date of birth would help the company to avoid targeting ads for age-restricted products at children, the Reuters news agency reported.
But Instagram said advertising was not behind the change.
"Asking for this information will help prevent underage people from joining Instagram, help us keep young people safer and enable more age-appropriate experiences overall," the company said in a blog post.
"We will use the birthday information you share with us to create more tailored experiences, such as education around account controls and recommended privacy settings for young people."
The information will help Instagram stop adult-focused advertisements for gambling, alcohol and birth control from being displayed to children.
But the company said that in the coming weeks it would encourage young members to switch on more privacy settings.
"Understanding how old people are is quite important to the work we're doing, not only to create age-appropriate experiences but to live up to our longstanding rule to not allow access to young people," Instagram's head of product Vishal Shah told Reuters.
At present, existing members will not be asked to provide their age because Instagram is not sure whether it would be too intrusive to ask, Reuters reports.
Additionally, the Facebook-owned social network will not verify the date of birth information it is given.
Criticising the plan, the child-protection charity NSPCC said in a statement: "Asking users to provide an unverifiable date of birth will do nothing in practice to protect children from harmful or age-inappropriate content.
"Forthcoming regulation will force platforms to go further and will require them to take steps to proactively apply additional protections to children's accounts by default.
"But first and foremost the emphasis has to be on ensuring that platforms are safe in the first place for children to use."
Facebook said: "We understand not everyone will share their actual age. How best to collect and verify the age of people who use online services is something that the whole industry is exploring and we are committed to continuing to work with industry and governments to find the best solutions."
Nobody will have their date of birth publicly displayed on their Instagram profile.
Instagram - along with its parent company Facebook - has long been under fire over its failure to enforce rules that say users should be 13 or over.
It is thought millions of underage children are on both platforms.
While Instagram is painting this new policy of demanding a date of birth as a child protection initiative, it will not verify that young users are telling the truth about their birthdays.
What it could do is enable the company to sell more advertising for age-sensitive products such as alcohol.
At the moment, the company has to be ultra-sensitive about making sure such adverts are not seen by children.
Now that it knows - in theory at least - how old its users are, it should be able to target ads with greater confidence.