Instagram Teen Accounts were set up in September 2024 to offer new protections for children and to create what Meta called "peace of mind for parents".
The new accounts were designed to limit who could contact users and reduce the amount of content young people could see.
Existing users would be transferred to the new accounts and those signing up for the first time would automatically get one.
But researchers from 5Rights Foundation were able to set up a series of fake Teen Accounts using false birthdays, with no additional checks by the platform.
They found that immediately on sign up they were offered adult accounts to follow and message.
Instagram's algorithms, they claim, "still promote sexualised imagery, harmful beauty ideals and other negative stereotypes".
The researchers said their Teen Accounts were also recommended posts "filled with significant amounts of hateful comments".
The charity also had concerns about the addictive nature of the app and exposure to sponsored, commercialised content.
Baroness Beeban Kidron founder of 5Rights Foundation said: "This is not a teen environment."
"They are not checking age, they are recommending adults, they are putting them in commercial situations without letting them know and it's deeply sexualised."
Meta said the accounts "provide built-in protections for teens limiting who's contacting them, the content they can see, and the time spent on our apps".
"Teens in the UK have automatically been moved into these enhanced protections and under 16s need a parent's permission to change them," it added.
