Yes, lots of under 13s are using social media platforms like TikTok. https://www.weforum.org/agenda/2022/10/children-digital-safety-social-media/ #medialiteracy
How can we improve online safety of children who have adult social media accounts?

The UK’s communications regulator Ofcom has found children are faking their ages to access social media platforms, raising online safety risks and exposure to harmful content.

World Economic Forum