Excerpt from this article:
The reason that most social media platforms have set 13 years as their cut-off point is a US law called Coppa (Children’s Online Privacy Protection Act), which dates back to 1998.
It mandated that online services would have to seek “verifiable parental consent” from younger users and would then be restricted as to how they could use the data.
Many apps subsequently decided it would not be worth their while.
Facebook was the most popular with under-13s, with 49% claiming to be users.
Instagram was used by 41% of 10 to 12 year olds and the company said “keeping the community safe” online was its “number one priority”
Among 16 to 18 year olds, two in five had used social media to spread gossip and a quarter had used it to say something “unkind” or “rude” to someone else online.
Related: Here is an article about a platform designed for kids under 13.